I say no for so many reasons. The main reason is Bettas are not active swimmers, rainbows are very active and large. It will stress the Betta. If you have a 55g or larger, small fish that school actively like neons or white clouds don't seem to stress them out so much. However, I've had neons nip at my betta tails in small tanks. People argue that white clouds can't be in 76 degree water and have a long life, but I've been doing it for years and had no issue.
My most successful pairing with male Bettas has been white clouds. But only in 29g or larger. Anything smaller and the Betta is better off alone IMHO. Because the Betta might harrass them and eat them in a smaller tank.
