I would suggest if your able to with a tank that small would be to consider returning them to your lfs or seeing if they would trade them in for credit towards something more suitable for your tank. Rather than saying your overstocked right off the bat, try giving them alternative options such as trading in or rehoming them if possible. I don't want to come off as rude or arrogant but I see to many threads that don't give positive alternatives.
May I suggest looking into nano fish or smaller options such as celestial pearl danios, harlequin rasboras, chili rasboras and other types of nano fish that are much more suited for your tank size. Pygmy Cory's would be an option as a clean up crew as well.
Another thing with platys and Molly's is that if you added more either male or female they are live bearers and drop fry roughly every 28 days therefore your tank could be overstocked quite rapidly and you'd be in a pickle.
Hope this helps