I have heard of a Platy and Betta working together. I would suggest trying it since he is biting his tail and sometimes a larger tank helps a tail biter. The other fish may give him something to distract (not in a bad way) him from his tail. I would choose a day that you can keep an eye on the tank for the first several hours and see how they react to the move. Bettas normally will not try to bully the other fish if they are already there first.