If the mollies are going into the 10g for certain, then I would suggest:
1. Not a betta. Though it's by no means certain, bettas can mistake colorful, long finned fish like mollies for competition and fights can ensue. It's probably not worth the risk.
2. Not corys or otos. Mollies do best in water that's had a small amount of salt added. Even if you don't add salt, if the mollies start getting sick or sluggish, a little salt can often help their health. With corys in the tank, you can't add salt. They simply don't tolerate it.
3. Option 1: All livebearers. Mollies, Platys, maybe a guppy. Male Endler's Livebearers are quite small but really colorful.
4. Option 2: The 2 mollies and a school of 5 harlequin rasboras or other SMALL schooling fish.
Remember, 10g isn't a lot of tank, so don't overstock. Live plants could flourish with the strong lighting you have and would help keep the water stable (perhaps allowing for a small addition or two in a few months). Plants also help your fish feel happier and more secure.
Hope this helps!
Good luck!
