A lot depends upon the fish. I am such a scaredy cat that I darent put my bettas in my community tank. They have their own little batchelor pads! I know some people do keep them in communities though. I am sure someone will post and put you right.
When it comes down to it, bettas do best on their own. They are not sociable fish in the wild and neither are they when domesticated- its true their are some fish that are reasonably safe to put with bettas, although bettas personallitys vary a huge amount in how agressive they are, but in my honest opinion if you want a happy betta, just give him his own spacious tank with some interesting decor for him to look at and explore, on top of a good diet and clean tank, and he'll be very happy .
I keep my boys isolated cause they get nippy with other fish and even snails.
The girls are different, they have cory's and female endlers in their tank, they all seem to get on well enough. I tried a snail but caught one of the girls trying to nip it feelers to out it came.