Thing is, mollies can get kinda big for a 10g tank, no? I mean, don't most mollies reach to 3" (or even 4") at full adult size?
From the size of tank perspective, I would say a betta plus a school of small, peaceful tetras (like neons) would be the way to go. I've kept neons in the same tank with a (female) betta and never saw the slightest hint of "nippy" behavior from them.
And as for stocking, *once the tank is fully cycled*, you could probably do a betta plus a school of 8 or so neons. Maybe even more, as neons seem to be very low-bioload fish. (As opposed to most livebearers, which are veritable poop machines...I think the 3 swordtails I have produce more poop than the other 15 fish in my community tank combined!)
Or assuming you build up slowly, and have a good HOB filter (an AquaClear, etc.), you could probably do one betta, a small school of 6-8 neons, and a small grouping of maybe 4 (small) corys as a bottom cleanup crew. Would make for a very nice, active tank.