Would it be safe to add a betta fish to my current tank?

Howdy ya'll. I currently have a 20 gallon tank setup with an air pump and filter. I have a thermometer and the temperature stays in the high 70s. As of now I have 5 tetras, a feeder fish (some crazy girl put it on my doorstep and it sparked this pion), and a white molly. I notice that the white molly is a bit aggressive and picks on the feeder fish. It even nipped off a piece of Penelope's back fin. The point is, my molly (Imp), is a tad bit territorial around the some driftwood I have floating around, but currently I feel all the fish are healthy and safe. Point: Imp is territorial. I've been wrestling with the thought of adding a Betta Fish because I think they're awesome, but I don't want the two to quarrel. Are females more pive than males?? Please, any details or personal experience on the matter would be great. Thanks!