If you have no breeding or possible breeding that will cut down aggression. Also, if fish are added younger/smaller at the same time or around it, I feel aggression is lower too. If space and food arent compromised I believe aggression is lower.
But this isnt always true for some species, some are too aggressive regardless and end up needing whole tanks for themselves. Some people are selfish and try to throw fish in and let them battle it out, thats not the way to do it. If you dont wanna end up in this type of situation then dont get these type of fish. If you do, accept the fact it will probably be a tank with 1 fish only.
So its the question, do you want a community tank or a solo fish in a tank? Some people make bad decisions and get too aggressive of a fish, and try to force a community later and lots of fish are killed as the result.