You don't want to overload your tank, so you must be careful.
Tetras are out. They are known fin-nippers, simi-agressive, and many can have colorations similar to Betta which can cause fighting. Take my advice on this one as I learned it the hard way! They need large groups and even then, there's a risk.
Another female would be good. Many say they work best in groups of 5 or more. Keeps down the fighting
Corys are great with Bettas, but in a 10gal, you will overload the bioload even with small corys or otos (which actually have a larger bioload than corys!). If you ever decide to do a 10gal with 1 Betta or a larger tank, these are great!
Shrimp are lots of fun! Little comedians I call them. They play and goof around. Best of all, the have vertually no bioload! That means you could easily have a dozen cherries or ghosts in your tank with 5 female bettas! Just make sure they have places to had so your girls don't make snacks of them!
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