I have a heavily planted 3 gallon tank with 1 male betta, 2 ghost shrimp, and a nerite snail. Now I know that those are not fish, but they are tankmates. At first I worried that Fat Boy, the world's hungriest betta, would eat the shrimp, and he did seem to be stalking them, but things have ended up just fine. I'm sure the heavy plant cover was useful when the shrimp were new, giving them plenty of hiding places, before everybody learned to get along.
I have a second tank that will soon be a betta tank. Current inhabitants are cory cats. I don't actually know from experience that this will work out well, but I've read that it does. And I have high hopes. :)