You were actualy more likely to get fry from a male guppy to female molly crossing than from this male molly to female guppy crossing. Still, it's possible. Unfortunately, many of the fry from this sort of cross are not viable or healthy. Also keep in mind that a lot of the mollies and guppies available are hybrids in themselves (the mollies are crosses of all sorts of species p. velifera, p. latipinna, p. sphenops and the guppies are often endler's X guppy crosses). This just adds to the difficulty of hybridising the two.