Coriadora Catfish are great for any size tank with a Betta. In a 10 gallon you can put 4 in there. Try to get a few females as they will lay eggs on the side of the tank that are like Candy to the Bettas. They are also good at cleaning the bottom of the tank. I only once had a problem with a female and a Cory. She didn't like Him eating HIS food because to her, ALL FOOD IN THE TANK IS MINE MINE MINE MINE. So I had to seperate them cuz she wouldn't let the poor guy eat.
One thing about the female Bettas: They sure put a bigger priority on food then the males.