Platies and Swordtails are both in the same genus, Xiphorous (28 species total). I have a really nice book published by TFH that covers all the species in the genus. The commonly seen ones are the Green Swordtail (X. hellerii), Common or Southern Platy (X. maculatus), and Variable Platy (X. variatus). These three, and well the practically the whole genus, can and do interbreed easily. With these three though you'd probably have to work to find some that aren't already hybrids. Many of the colors commonly seen in them are a result of hybridizing.
Guppies and Mollies also share the same genus, Poecilia. Guppy (P. reticulata), Endler's Livebearer (P. wingei), Short finned or Black Molly (P. schenops), Sailfin Molly (P. latipinna), and Giant Sailfin Molly (P. velifera) are the common species. The three molly species can and will interbreed and again this can be another case where depending on what color/form you're buying you're already getting a fish that is a hybrid. Same can happen between the Endler's and the guppy. Some Endler's might already be hybrids with guppies. From what I read before Molly/ Guppy hybrids only occur through artificial insemination and never live to reach adulthood. I've also never heard about a cross between a member of each genus. As far as I know it's a myth.
