Of course - I have done so myself - it's just that this isn't a community tank so to speak. It would be a species tank for female bettas from what I understand and sometimes they can be aggressive towards one another and unpredictable. It's usualy easier to obtain compatible tetras as most bettas in shops are kept and sold seperately - even females - so suddenly putting them together can result in aggression. Also, if you cycle the tank with fish, you can add tetras few at a time. If you add bettas few at a time you'll get serious disputes every time a new one is added.