You can keep a betta with wcmm (I'm doing it right now) but you're going to compromise on one of their temperature needs no matter what. If you've only got the betta right now why not get some smaller schooling fish that will thrive at a betta's temp, not just survive? WCMMs are great but there are a lot of other great little fish out there too. Just figure out what you like about them and choose a fish that has similar qualities. My betta and WCMMs are doing fine together for now in a 20 long but it's really not ideal and I end up feeling bad about keeping these fish but having to compromise on their needs. It's up to you.