AngryPuppy, where did you get the idea that bettas are aggressive? I have found, over the last 55 years, that they are not very aggressive and are far more likely to be the victims in any confrontation than the aggressors. I find, through actual experiences, that they do great in any peaceful setting, not one involving things like barbs for instance. I have my present one in a nice peaceful endler breeding colony. He spends all day every day in this environment with no troubles at all.
At meal time he just swims along with those endlers to where the food is. The rest of the day he interacts with the endlers and their tank-mates, the bristle nosed plecs with no trouble at all. He also swims right through the canister filter's flow, that is required by so many fish in that small space. He acts, dare we say it, just like a fish.
This is my most recent picture of him at a youthful 2 years in my care.