General rule of thumb as far as how many fish go in a tank --

one inch of ADULT FISH per gallon.

So lets say you get a fish that is only 1/2 inch now but will grow to be 3 inches. You need to think long-run and just assume it will grow to be three inches. So instead of buying six baby 1/2 fish, you should only get 1 fish because it can get up to three inches in length.

10 gallons = 10 inch of adult fish and NO MORE.

Hope that makes sense??

NON AGGRESSIVE and NON FIN NIPPING fish will be good to be housed with a betta. I don't recommend going to WalMart because their associates aren't educated enough to be able to help you out (that, and their fish are always sick). I recommend PetCo or Petsmart where they actually have to take classes on aquatic care before even working in the aquatic department. I used to remember from my class what fish were compatible with a betta, but I forgot lol