In a 10 gallon tank, you have 10" of adult fish that you can put into the tank, without causing water pollution problems.
Your male betta takes up 3" inches of fish... that leaves you 7" to work with.
You could have a school of 5 fish that will reach 1.25" long as adults. Neon tetras come to mind. You could have 5 pygmy cory cats as well.
Besides adult size, you'll also want to be sure the tank buddies you choose can live in the same conditions as your betta, so everybody's happy.
Have fun choosing your tank buddies! :D