Neon tetras are good tankmates for bettas. Cory catfish will work too, but get small ones (panda, pygmy, or dwarf) to fit in the 10 gallon. Some bettas will bother their tankmates, others don't. An example of this is I have two bettas. one is a docile, friendly fellow named Waffle and the other is a very agressive, im-the-boss kind of guy named Martin. I added black mystery snails into both tanks. Waffle became best friends with his snail, always watching it crawl around and completely fascinated. Martin, however always picked on his snail, flared at it, and poked its antennae. It proves how adding tankmates is a trial and error kind of thing.
But with fish tankmates, I'm more worried about the betta being bullied than the betta being the bully. Sometimes, fish will nip their long, flowing fins. If you put them together and see signs of fin nipping, remove the betta to a new tank or if its only one troublemaker, remove that one instead. But many people are completely successful with having betta fish in community tanks. Just be prepared to have to take someone out.
