Most people on the forum will probably tell you that bettas are no good in a community tank. I disagree however, and think it really depends on the betta's personality and the other fish you have in there. Right now I have a community tank with Cardinal Tetras, Cories, and Ottos that used to house my male Polaris who died several months ago, and now houses my female CT, Artemis. Polaris was a very passive boy, and while Artemis has a little more spunk, she has only been curious as to her coinhabitants, and has never attacked them. Both have been very happy with their arrangement.
I won't say that this is true for all bettas. Some will attack anything that swims, or appears to swim. And while this isn't bad, a betta that does such would be best by itself. But if you have a fairly good tempered boy or girl, it can be done. If you do decide to add a betta, I would make him or her the last inhabitant you add, and then would watch the tank closely for a few days to make sure that everyone's getting along okay. Have a backup plan too incase things don't go as planned.
Good Luck!
~Kelly