It's all going to depend on the tankscape. three pair may be pushing it. If you plan the tank with caves and plants at each end leaving the swimming spaces in the center area. The more likely your pairs will avoid each other. Things may be more dicey when they are breeding. As for other stock. I think you would be better off under stocking the tank. Choose fish that aren't bottom dwellers and faster moving. Medium tetras or barbs. Danios would also work I think.