It really all depends on the temperment of the betta in the tank whether he will allow other fish to live with him. I have one with 7 white clouds in a 10G, and he does perfectly fine. I also tried an otto with my betta in a 3G and he tortured the crap out of him.
Good luck! Neons or white clouds should both be wonderful if your betta accepts them. Though white clouds are technically cold water, mine are perfectly fine with my betta in 74-76 degree water.