Hey! thanks for the question. Unfortunately, I think the best you’d be able to get in an 8-gallon tank is probably going to be shrimp or snails. Mainly because any other fish you could add are schooling fish, so keeping them in groups would overstock your tank.
If you could swap the tanks the betta and the crayfish are in around, then you could put some corydoras in the tank with him and quite a few species of rasbora as well!
But if you plan on keeping your crayfish in the 10 gallon, then you could also add some guppies to it, to give it a lot more color!