Certainly can be done. It works for sailfins, and for most all hybrids available and as the gross majority of mollies for sale are hybrids it is not an issue. Mollies, like most livebearers are amazingly flexible, and are found naturally in brackish water, but full sea is no biggy. They actually have some omportant morphological features like the gill rakers that are marine rather than frsh in design, and that is important as the gill rakers are used to control body fluids by excreting urea and water.
They are killer alage eaters, for all the real problem hair algaes. It should takes a day or two to get them up to full strengh SW from FW