It is highly unlikely that any attempted mating between a guppy (Poecilia reticulata) and a platy (Xiphophorus sp.) would be successful, as they are too distantly related...however, guppy x molly hybrids (usually called 'muppies') and platy x sword hybrids are not uncommon.
HAHAHA! Does this sound familiar! i have a male guppy who shows NO interest in the 2 female guppies, but chases 2/3 female swords all over the place, and my largest female molly (of 3). I have 3 pregnant ones - one guppy, one sword, and one molly, - guess who he chases! Silly fish! Stick to your own kind at least. She's 2-4 times his size.