mollies have been proven to due significantly better in full marine if not at least SW environments. in fact, most are regularily found in SW in the wild.
When acclimated to SW, their rate of illnesses drops 90%.
just my two cents, but bearing this in mind, i wouldnt put them in FW for long.
btw, the whole "livebearers that could work in a small tank and are easy to breed", this could be a blessing and a curse, as most livebearers have a habit for filling up tanks to the brim.