Male bettas will be especially aggressive towards female bettas, more so than other fish. Its part of their natural breeding cycle. Even breeders don't keep males and females together, they're acclimated to each other then introduced for the breeding process then separated.
Good betta tankmates in my experience (as in I have personally tried them) are Cory cats, neon tetras, small catfish, platys, or giant danios. Don't try barbs for the tendency to be fin nippers (mainly the tiger barbs, some Cherry's are peaceful if kept in proper m/f ratios) or fancy guppys as bettas will often mistake them for young male bettas and attack them.