I have to second the danios - beautiful little fish - 5 or so long-finned zebra danios would add so much activity to your tank and are attractive and have lots of character for such a cute little fish.
Harlequin rasboras are, however, the best schoolers.
Hatchet fish are nice but can be bullied and are rather sensitive.
I'd avoid tetras because they may not like your water (do you use any salt with the mollies BTW?). If I had to suggest a tetra though, black phantoms are great and are very hardy for a tetra. The males also display interestingly to one another.
How about 5 or so cories? If you don't use salt... They are outstanding little fish and do best in large groups. They are adorable and so much fun to watch.
Bristlenose plecs are cool too. One of those or some otos would be nice additions but only if you don't add salt and your water isn't too hard.
A good top dweller are gouramies - in a 20 gallon you could go for 2 dwarfs or 4 honeys (make sure you don't get more males than females though - or the females will be chased and harassed). You could also get a single thick-lipped gourami or one pearl gourami or one indian/banded/striped gourami.
Some platies or swordtails would also make a nice addition. Especialy if you have hard water. Guppies would work but mollies and guppies are quite similar so...
Though if you want to stick to livebearers, you could always go with a different type of molly.. I think they call them gold dust mollies at my superpet, but they're such a gorgeous vibrant yellow, and their babies are adorable! 
