K, I have had reports of bettas in community tanks that
- Get Killed by Angels (they don't mix - Angels will gang-up on bettas)
- Fight or Kill or get Killed by Gouramies (both top/level territorial)
- Kill Guppies (males especially like to hunt down guppies at night)
- Kill Tetras (again, mostly at night when they can sneak-up)
- Kill each other (obviously, but even females fight)
- Starve or get sick from other fish eating their food
- Loose fins from fin nipping (ie: barbs, some tetras)
Those are some examples of the more common problems reported to me when people ignore my suggestion of not mixing bettas with other fish.
I think bettas are just designed to have their own space. In a wild environment I am sure betta's would not tolerate any other fish in their territory, except a female betta duing breeding ONLY.
Age plays a big role, the older they get, the more space they seem to want. Sorta like people, first we can share a bedroom, then we want our own room, and eventually we want our own house.
Now, I think bettas don't mind 'visitors', as long as they do not invade their space, so divided or nearby seperate tanks are more then welcome.
But thats just my opinion.
(300 betta's later & still Learning!!!)