I thought mollies only grow to be 3-4 inches? All of mine are balloon mollies, and because of their spinal deformity they only grow to 2" or less.
Still, for regular mollies, having 6 in a 15G is pushing it, if not outright overstocking, for sure.
I agree with the suggestion of finding a different home for one of the males. I've got just one male in my 38G tank with 3 adult females and a bunch of fry, and I have no fighting or bullying problems whatsoever. The male has no competition and spreads his attention around pretty equally, so everybody's happy.