I currently have 16 mollies in my tank, only 3 of which are male, (4 dalmation, 1 marbled, 2 creamsicle, 1 red, and 2 silver, 2 juvis and 4 fry) also with a GBR, 3 bettas, a flame red dwarf gourami, a couple of green corys and 1 neon tetra who i need to get some more friends for.
I would not recommend mollies for your tank. With your filtration, do I think your tank could handle it? Sure. Your problem comes with the fish. I wouldn't put more than lets say 2 maybe 3. If you go all male they will get frustrated eventually and try to mate with whatever they can get near. If you go all female, there is a chance they can be pregnant from the store. One of my female creamsicles came pregnant and the 4 fry I now have are the result.
Have you thought of female bettas? I was directed to them by some people on this forum. They don't look all that great in store, but once they are home and in a tank and not a cup their colors really come out.
Actually, when I have a school of neon tetras one of my bettas will swim with them and change her colors to match. Really amazing to watch.
Well this was just my 2 cents! Hope I helped somewhat!