Betta's work with many types of tropical fish (I keep them in community tanks myself), but they are territorial and need space, so nothing less than a 10 gallon to keep them with other fish, and there are certain types of fish that don't work.
A gourami is an example of a fish that does NOT work with a gourami. Both are territorial and they will fight for space. Keep only one or the other or one will just die of injuries or stress.