When i bought my betta he was in a tank with about 30 mollys and his fins were ok and i dont think he'd been aggressive towards them. He's now kept on his own though as he doesn't get on with my fish... It depends on your fish's temperment i guess, you could try and see how it goes...
The only fish ive found that always seem to get on with my bettas are swordtails. Ive never had any problems with my female swordtails and bettas!!