i agree and dissagree to a certain point. You say a fish will be less aggressive if it has more tank mates and overstocking is happening. I think it really depends on the type of fish , his idividual personality , the personality of the other fish , the species , the tank , the decor , stress levels, filtration , feeding , and water params ....etc.
BUT
You ever think maybe your fish quit trying to attack each other because they are too stressed out to do so? They use to have a home worth fighting for , now they dont even care for it. Just another overcrowed miserable place. Just like the LFS you "rescued" them from.
Are you now looking out for your fish , and their personal pleasure ? Or looking out for your own , and what you would like to keep in the tank by pushing your fishs' comfort over the edge?
Don't take this as a personal attack anyone , i just thought it would be fun to play devil's advocate. Just another opinion that hasn't been mentioned. I think somtimes having a tank thats overstocked for looks and personal pleasure can somtimes be sick. I agree with managed overstocking , but in the end if you can manage the overstocking , why not just manage the stocking?