There are several posts running right now about what to put in with Bettas. They are not particularly community fish. Most Betta tanks are built around the Betta and have very few other fish in with them. A few candidates may be (depending on the size of the tank) Cories, Otos, and Snails. Perhaps some of the others can come up with other fish, but the list will not be a long one. They are not sociable fish and don't do well with them. They are a "people" fish. They really enjoy our company more than other fish and are content to be alone or with a very limited number of neighbors. Other fish, especially those who move around very quickly, tend to make them nervous.
If you are talking about a small tank (5 or 10 gallon) there will not be room for many of the above even as they are fish who love in groups of 3 or more. Cories are too big to put into a 5 gallon with a Betta.
I have a Mystery Snail in each of mine and 2 Zebra Nerita Snails in the girls tank beside. They do not add much of a load on the bio system and they do a good job of cleaning the tank and my nitrates have been lower since I got them.