People generally favor bottom dwellers because they balance out the tank the most since they occupy the opposite part of the tank than bettas and they are the most likely to get along with a betta. However, there's no reason not to add another kind of fish. Here is the criteria you should consider:
-size and space requirements; will they fit with a betta in your size tank?
-activeness; very active fish will bother the calm betta
-nippiness; fish that nip will shred your bettas fins very quickly
-ornamental finnage; any fish with long flowing fins will be picked on by your betta, for instance, threadfin rainbowfish would be a poor selection
-water column occupation; bettas are territorial, so they prefer fish that will stay out of their way. Other fish that prefer the top region of the tank is a poor selection because your betta will want them out. Gourami, for instance, would not be a good tank mate.