With a betta I would always exercise caution regarding very small creatures that he could eat, but even more, regarding other fish nipping at his long brightly colored fins.
Generally bettas will do best in a very peaceful slow moving tank. They do well with other fish who will generally leave them alone such as cories, neons, otos, plecs, white clouds, rasboras, etc. Fish that are similar to him, such as gouramis, paradise fish, etc (labyrinth fish) can potentially be trouble for him as one he or they or both will display very aggressive behavior towards each other.
There are no hard and fast rules with fish of course, as I've even heard of two male bettas who once managed to join each other by jumping a divider and living in perfect peace! (Yes the owner was an idiot and let them stay together for over year until one died of natural causes....though I'm glad he did because it showed how weird things can be sometimes. )
Then I've heard horror stories of a male betta being beaten to shrivels by a female betta who then went on to destroy his bubblenest almost in a fit of shrewish rage!
I would suggest simply keeping him in his own little place or in a community aquarium with only the fish that I mentioned above. *Though there are many others I can't think of.