the hobby is not about (or should not in my opinion) be about what fish is the meanest, baddest, toughest, most aggressive and blah and blah. proper care of your fish means not allowing the aggression and battling you seem to love, to happen in the first place.
think about this: a fish tank is an artificial environment. aggressive confrontation which results in battles, is NOT something that routinely occurs in the wild. in any native environment, there's alot of room for the fish and when one gets in anothers way, one of them can leave. in your tank, the lowest fish on the totem cannot leave ... there's no place to go.
in the wild, fish occupy a specific ecological niche, have different feeding requirements and occupy different locations and strata within any lake or stream. this is how they all "get along" in the same lake. they specifically avoid battles to the death and you can be sure they couldn't care less who is the baddest *** on the block. the only exception to this is during spawning, when males will stage mock battles in order to determine who is the best genetic fish to mate with a given female. this is good for the species since it helps to foster the strongest, best looking fish.
i think it's safe to say that generally, folks tend to keep too large fish in too small tanks and/or too many fish in any sized tank. this only makes the aggression problem worse.
the interraction of a mixed community tank and/or the behaviour of a single species is the single most important factor to consider when keeping multiple species. you want the fish you're keeping to have the ability to exhibit "normal" and functional behaviour. normal behaviour is important. abnormal behaviour results in stress. stress leads to disease. disease can lead to death. for example:
1. if species (A) does not get along with species (B), then these fish don't belong in the same tank no matter how few fish you have or how large the tank. "getting along" is based on the visual and behavioural Q's that each fish species is genetically programmed with.
2. if species (A) has specific food or water chemistry requirements which are completely different from species (B), then these fish don't belong in the same tank no matter how large it is.
3. if species (A) is aggressive and defends a territory of 2 square feet, then you cannot expect to successfully keep more than one of these fish in a tank which is only 2 square feet. convict cichlids are an excellent example of this ... this little fish aggressively defends territories of about two square feet give or take. if you have a tank which allows only that much room and no more, the convict will defend the entire tank to the detriment of any other fish (no matter how large it is) in that tank.
fish need room to swim without having the rest of the tanks inhabitants "in their face" continuously. when crowded, fish exhibit stress syndromes that result in poor color, improper fin form, insufficient metabolic development, do not exhibit proper musculature, do not develop properly functioning organ systems and most importantly slowly lose their inherant resistance to disease. this results in a significantly shortened lifespan and along the way, lots of diseases for which the poorly conditioned fish is a good target..
think about your local lake or river --- the fish are free to inhabit whatever space suits them biologically. if it gets crowded by their standards, some will disperse and move to other areas where they again have the space they need to exhibit functional behaviour.
it's difficult to allow for that "space" in your tank -- the fish have no escape within the confines of your tank so it's up to you to insure that the fish have that space in the first place.