Try to stay away from fish with long or wavy fins, the betta may nip at them. But also remember bettas can easily be picked on also, they are slow swimmers, they really float more than swim. I had one in with a school of danios and they attacked him constantly. I have had success keeping bettas and white cloud mountain minnows together though. I think those two would be a good combination not only because of compatibility, but like bettas WCMM's do fine in a non heated tank. Plus they are small and you can have more of them in a tank then other fish, you could have a nice school of them.
Caveman