It depends on the betta really. You'd be best off sticking with something like cories or a lrge group of kuhlis, but if you went with a group of female bettas instead, they not only would be better with the current but are generally more peaceful.
I have a Jewel Rekord 120 with a male betta, 9 harlequin rasboras, 1 molly, 7 panda corys and 6 kuhli loaches and he is fine with all of them. I've got a sand substrate and real plants and it's a nice peaceful community.
To be honest, I don't know a great deal about bettas, apart from a lot of reading, but a lot of what I have read says that keeping a betta in a commuinty depends a lot on the individual betta's personality so maybe I got lucky and got a peaceful one.
You might have enough room to put in the required number of gals for a sorority tank, but 15 gallons isn't a lot. I think a something to consider when buying for bettas is don't buy fish that inhabit the same area of the tank as they do, ie. no top or middle dwellers. And of course, watch the temperment of the betta!
I've never had issues with a very aggresive male in my 20 gallon becuase the only other things in there are 6 black kuhli loaches and three snails who are a lot bigger than he is. I had to move the marisas (ramshorn) snails to the other tank because this betta ate their feelers. I have another male betta in a seven gallon who doesn't bother the snails, the new ADF, or the ghost shrimp.
You could seperate the tank into different compartments and keep a few male Bettas, the compartments could be aquascaped the same as you would normal full sized tank/s.
If your heart is really set on keeping Bettas that is
Regards
BigC