Alot depends on the individual betta. Some bettas are very mellow and could care less about other fish while some bettas won't tolerate other fish. The one fish bettas will generally always tolerate are cory cats. Since the cory's are bottom dwellers and not brightly colored the bettas just tend to ignore them. Bettas and dwarf gourami's are a deadly mix. Betta's are more prone to become aggressive with other brightly colored fish and fish with long flowing fins. Plus Dwarf gouramis and bettas occupy the same tank space staying alot towards the surface which makes matters even worse. Then on the other hand certain fish play heck with nippying at a bettas long fins.