There are a number of types of aggressive behavior in dogs and it is possible for dogs to exhibit more than one type of aggressive response at the same time, in some instances. This can make it very confusing to sort through the possible diagnoses and treatment options. Whenever it is possible to do so, it is best to seek the advice of a certified animal behaviorist or board certified veterinary behavior specialist when dealing with aggression. The earlier this can be done, the better. Unfortunately, that is not an option in many areas of the country since the number of behavior specialists is still low. 

Tail wagging in dogs is not a sign of friendliness in many instances. It is an indication that a 
dog is prepared to engage in a social response -- but that can be an aggressive response or a friendly one. Sometimes it shouldn't be taken as a sign that she initially feels like she wants to be friendly and then changes her mind. It should simply be viewed as a signal that she is 
aware of the other dog and intends to interact in some way. 

It seems strange that a dog that is frightened would chose to attack rather than to run but that is frequently what happens when dogs have fear based aggression towards other dogs. It is hard to be certain, but it is possible that the reaction to smaller dogs may still be fearful aggression rather than predatory behavior. I have seen a number of large dogs react just as fearfully to smaller dogs as to bigger ones. These types of determinations are hard to make without the opportunity to see the interactions (and since I'm not a behaviorist they would still be open to interpretation if I made the diagnosis). In addition, some dogs with aggression that really appears to be fearful in origin are actually territorially aggressive or may even be exhibiting an inappropriate form of dominance aggression. 

There are some general steps to take when dealing with a dog that is aggressive in any form. Spaying and neutering help often enough that these procedures should be considered. It is highly unlikely that spaying or neutering will totally control aggressive tendencies, though. It is important to teach a dog at least one or two commands and to gain enough control of the dog to be able to use these commands reliably. If this can be accomplished it is often possible to distract the dog when it appears likely that a situation that might elicit aggressive behavior is developing. Most behaviorists recommend using a "Gentle Leader" or halter 
type collar to obtain even better control of aggressive dogs. These collars work like a halter and allow better control of the dogs's head. Many clients who would lose control of a dog with a leash attached to a regular collar (or even a choke collar) can control their pet when it is wearing a Gentle Leader halter. Plus the portion of the halter than goes around the nose tightens when it is pulled on, making it possible with quick action to prevent the dog from biting in some cases. This can't be relied on, though. 

Fearful aggression towards other dogs often starts with barking and nervous behavior at the sight of another dog. Discouraging the barking can help to stop the escalation of the fear and aggression so your current behavioral modification attempts in the car may help a lot. Reassurance may be harmful in the treatment of fearful aggression if it is carried too far. It is 
important not to reward the dog for fearful behavior by petting or phrases like "good dog" that the dog may recognize as cues that the behavior is appropriate. It may be better to stick with calm reprimands or to try to distract the dog by using a command such as "sit" or "quiet" or "stay" that you have previously been able to teach the dog to perform reliably. 

Keeping a fearful dog out of situations in which fear is generated for a few months can sometimes help a great deal, too. This seems to be especially true in dogs that are adopted from shelters or rescue organizations and have not had time to adjust to a new home after the stress of losing their original homes and possibly going through several moves in a short period of time. Once in a while, just having time to get secure in a new home and having a break from reinforcement of the fear is enough to allow a dog to let go of the fear and begin to react to other dogs in a more normal fashion. 

Letting the dog see other dogs at a great distance (whatever distance is necessary to keep signs of nervousness, barking, etc. from occurring) while working with the commands your dog knows in order to provide a distraction can be helpful. Gradually decreasing the distance to the other dogs -- often over months --- can often be accomplished. It may eventually be possible to even allow interaction in controlled circumstances but here is 
another spot in the training when having the help of a good behaviorist can be very very helpful. As you point out, it is hard to find a dog and its owner who are enthusiastic about subjecting themselves to the possibility of an aggressive encounter. A behaviorist can help determine when it is appropriate to consider this interaction and may also be able to help with finding a dog who can handle the encounter. 

In some dogs, behavioral modification will work better if anti-anxiety or anti-depressant medications are used in conjunction with the training. It is very important to remember that these medications just lower the threshold for aggression to occur in some dogs, rather than producing the desired calming effects. Use caution when medications are used. At present, 
amitriptyline (Elavil Rx), clomipramine (Clomicalm Rx) and fluoxetine (Prozac Rx) are the most commonly recommended medications for modifying aggressive behaviors. 

If your vet owns a copy of Dr. Karen Overall's book "Clinical Behavioral Medicine for Small Animals" there are good descriptions of the various types of aggression and also pretty detailed directions for desensitizing dogs to various situations, including the presence of other dogs. Dr. Ilana Reisner wrote an article that I think was published in either Dog Fancy or Dog World several years ago on aggression between dogs that was also a good 
source of information. If you get either of these publications and save them you might look back at the older issues to see if you have that one. 
There is also a book I have not read but which is written by authors (Landsberg and Hunthausen) who seem to give reasonable advice, titled "Handbook of Behavior Problems of the Dog and Cat". I also like a lot of Dr. Ian Dunbar's advice and I think he may have a video out on inter-dog aggression (I saw an ad for new videos by Dr. Dunbar but can't recall clearly the titles). 

Large dog attack on small dog - tragic results 

Attacks by a big dog on a small dog have the potential for injuries to the kidneys to occur in a number of ways. 
Big dogs often shake little dogs during an attack. This may fracture a kidney or disrupt its blood supply, although this is unusual. 

The tooth may penetrate into the kidney or into the tissues surrounding the kidney and cause damage. Retroperitoneal abscesses occasionally occur after dog attacks and may surface weeks after the initial attack as the infection slowly builds in intensity. 

Big dogs have tremendous crushing power in their bite and they may crush tissue such as muscle or skin and cause it to die. When large amounts of tissue die in the body it is hard on the kidneys and the other organs as they try to aid in the cleanup of the dead tissue. Organ failure during this process can occur. 

Shock or blood loss from an attack can lead to hypotension (low blood pressure). When blood pressure is low enough, the kidneys are damaged. In a pet with marginal kidney function prior to the attack shock can be enough to set off full blown renal failure. 

Infection of the kidneys from bacteria introduced into the bloodstream or other tissues at the time of the bite and then spread to the bloodstream can lead to kidney failure days after the initial bite. 

A dog with hypoadrenocorticism (Addison's disease) prior to the initial attack that has not been diagnosed previously may die or suffer aggravated secondary effects such as kidney failure due to the pre-existing disease. Addison's disease can have very subtle signs in many dogs and it is not unusual for it to go undiagnosed until a crisis occurs. 

Blood clots formed due to trauma may break loose and damage any organ. The kidney is pretty susceptible to damage by blood clots due to its rich blood supply and the way the blood flow is directed through the kidneys. Bleeding disorders associated with major trauma or shock may lead to kidney damage as well. 

Due to the potential for severe internal damage even in bites that do not seem to be especially serious based on the initial signs or skin injuries there are veterinarians who advocate surgical exploration of every puncture wound found after a dog attack. This is expensive and it subjects the dog to anesthetic risks which may also be higher due to trauma so there are also a number of veterinarians who feel that it is better to try to stabilize the patient and assess the damage more as signs develop. We have had success and failures using both approaches and I honestly do not know which is best in the majority of cases. 

When bite wounds start to develop secondary complications it is not unusual for a case to go from bad to worse to really awful rapidly. Attacks by big dogs on little dogs are true nightmares from a diagnostic and treatment standpoint. 

This attack should be reported to the animal wardens or appropriate authorities, as you indicate. In some localities it takes two or more confirmed reports of attacks before any legal consequences are possible. Since there is the potential for this to occur again it is important to report this instance. 
  

Aggressive behavior 

It is not possible to ever fully trust a dog that has exhibited a tendency towards aggressive behavior not to revert to that behavior. Virtually any dog can be induced to aggression given the right circumstances and dogs that already have exhibited aggresive behavior are easier to incite. So you will have to be careful for the rest of his life. You sound dedicated to him and this may be acceptable if you can reach a "mostly trustworthy" situation. It is usually necessary to constantly and consistently apply behavioral modification techniques that work and to keep this up lifelong, even when they work well. So this is a long term commitment. 

Who started the fight? Interdog aggression 

It is not unusual for interdog aggression to occur when a new dog is introduced into a household because territorial and dominance issues need to be resolved between the dogs. 

It may be best not to leave your dog with another dog when the dogs appear to have reached a point where someone feels threatened and is responding aggressively. 

Prozac for Aggression 

I have corresponded with Dr. Ilana Reisner, a board certified veterinary behaviorist, about the use of these medications. She has been trying them in some cases of aggression and feels that they can be useful. 

There are several behavioral modification medications used to treat aggression in dogs. Prozac is one. If I understand the references to it correctly, this has been used mostly to treat dominance aggression. Lithium carbonate is also used for dominance aggression by behaviorists, for some cases. This sort of aggression is usually directed at family members and can occur quite suddenly. Since that isn't the problem you are having, there may be other approaches that might help more. However, I am not a behaviorist and can not claim to be up to date on the latest treatments. So it is also possible that these medications are being used for other forms of aggression and may be useful. 

For fear based and sometimes for territorial type aggressive behavior, the preferred medications seem to be anti-anxiety drugs. Among these would be diazepam (Valium Rx) and amitriptylline (Elavil Rx). These medications can reduce anxiety and make a dog a little less likely to respond aggressively to frightening or stressful situations. We have tried amitriptylline for several cases of aggression and feel that it has been helpful in one or two cases only. Most of the time, some sort of behavioral modification is necessary as well. 

A good physical exam and general lab profile to rule out underlying problems is always a good idea when assessing aggression. The next best step is consultation with a veterinary or certified animal behaviorist, if possible. In many places, these people are not easily available for consultation, though. A really knowledgeable trainer can be a great help but it is extraordinarily difficult to evaluate trainer's abilities. Assuming there is not a physical cause and that you have exhausted behavioral modification techniques, you are left with muzzling Crom when walking him, which won't help with the lunging but might make people less nervous about him; use of a Promise or halter collar to give yourself more control (this worked very well for our Rottweiler who liked to lunge at people); and finally, medical therapy. 

Your vet may be able to at least consult with a veterinary behaviorist. This has helped us on a couple of occasions when the owner simply couldn't make the trip to the behaviorist. 
  
  

Acepromazine 

Acepromazine is one of the most commonly used tranquilizers in veterinary medicine. It is a phenothiazine compound. Its mode of action is only partially understood but it involves blockage of dopamine nerve receptors in the brain. It causes tranquilization and also has an anti-emetic effect. This makes it especially useful for treating car sickness, since that is often a combination of fear and motion sickness in dogs. 

The recommended dosage for acepromazine is 0.25mg to 1mg per pound of body weight. In most cases it is not necessary to use the higher dosages. That is not true for use in trying to control fear based aggression. Acepromazine is considered to be very safe. The average toxic dose is significantly higher than the recommended dosage (at least 20 times the dose). Despite this, acepromazine does have some significant effects that must be considered. It can cause hypotension (lowering of blood pressure). This effect may be exaggerated in Boxers and there have been anecdotal reports of death of Boxers after the use of acepromazine. In addition, acepromazine seems to make it easier for dogs with seizure disorders to have a seizure. This medication should not be used near the time of dipping or treatment with organophosphates for flea control. 

Acepromazine doesn't have any pain-killing effects. Many dogs seem to be able to will themselves to overcome its effects, at least temporarily. This makes it less than ideal as a drug for dealing with aggressive or fearful dogs but there have not been better alternatives for medicating prior to the visit. It works often enough that many vets will try this approach first. We do this when we think it has a chance of helping make an office visit go easier. We just remember to continue to be very careful when examining the dog. 

Another approach is to use faster acting medications after the dog arrives at the hospital. I have heard from several veterinarians that Telazol, an anesthetic agent, works very well to facilitate examination of aggressive dogs. It is supposed to be injected but despite that, it has proponents. Some vets have used it orally by just squirting it into the aggressive animal's mouth. I haven't tried this medication personally. We have used acepromazine and oxymorphone combinations with good effect. The oxymorphone provides pain relief as well as sedation and that seems to relieve some dogs that would fight acepromazine alone. Oxymorphone can be injected in the office after premedication at home with oral acepromazine. 

We see some fearful dogs at home instead of the office. For some reason, this works very well with some of these dogs. It definitely isn't a "cure-all" but it can lessen the strain of the whole procedure on everyone when it works. 

Some dogs do get better from year to year as they come in our office. After a few visits they seem to adjust to the scariness of the office or just resign themselves to the inevitability of the visit. Sedation in the early visits does sometimes seem to help this process. Unfortunately, it doesn't always work this way. 
  

Biting Dog 

Prozac may prove to be beneficial in some cases of aggression due to seritonin sparing effects BUT you really need to consider finding a specialist trained in behavioral therapy with this sort of problem. A certified animal behaviorist or board certified veterinary behaviorist would be the best choice. Please ask your vet to help you find someone who can help you. You may be able to find a list of behaviorists certified by the Animal Behavior Society at your library. 

Chasing mailman 

You have to do something to restrain your dog. It is just not an acceptable situation to have him chasing anyone down the block. Fencing works best for allowing him a lot of freedom but not enough to get in trouble. Keeping him inside and walking him only on a leash is an acceptable alternative. It is more trouble but it gives you an interactive event several times a day, which dogs generally like. 

Aggression is best treated by someone who can evaluate the situation in person. In many areas there is not a certified behaviorist to go to but if you are lucky enough to have one in your area, please consider this option. It is much better than trying to deal with this on your own. Your veterinarian may be able to help you find a veterinary or animal behaviorist who can help you. If not, your vet may feel comfortable in evaluating your situation and helping out. 
  

Canine Aggression-chasing 

The best solution to this problem would probably be to put up a fence around your yard. Chasing behaviors are very difficult to stop in dogs. It might be less expensive to have a fence than to keep paying for problems not having one cause.