Tank mates for a Betta, 9 Neon Tetra's 3 oto's and 3 Platys?

3 or more Corydoras would fit nicely in your tank, provided you don't have a TON of other fish in there... they are typically small, very peaceful fish. A snail or two could also be nice additions. In my experience, different types of livebearers like mollies or platys tend to get along with the friendlier betta fish, but it could be hit or miss.