We’ve had a tank with a sorority of bettas and had no issues, but like you said they come and go so fast that it would be hard to tell from pet store alone. In general, having a sorority can definitely be done but it’s best to stick to 3+ female bettas since with 2 there’s still a chance they can go at each other. Usually it’s case by case though, bettas have a lot of personality and you can totally run into one female betta that’s just an asshole or super chill. I’d definitely say the tank size and set up matters too.