I have corydoras with my betta, they like to be in a group and require 10gals or more. If you get them, try to get 3 or 4 of the same kind so they can school, they are really entertaining.
When introducing them, be careful. I took out my betta, re-arranged all the decor and changed the water in the tank. I then put the cories in first let them get situated, and then put the betta in. This makes the betta think that he is in a different territory.
My betta doesn't mind them at all, which I find weird since he tried to eat the snails I got. (One snail floated (which happens, they are still alive) then he tried to take a bite.) Once the snail got bit, it never came out of it's shell, and if it did my betta would nip at its feelers. It eventually died..
African dwarf frogs are so cute, but I talked to someone and they told me that if you get them and you have hiding spots in your tank, you will never see them again. Another person at petsmart told me they are very picky eaters and they had to feed them through a turkey baster type thing because they wont come out for food and their sight is very bad.
I recommend Corydoras. I was surprised how well my betta got along with them. He flared at first, and chased, but cories are very fast and he gave up quickly and now doesn't mind them at all, they'll even swim right into him and he wont care. They are also very entertaining and my albino cories are very very active.