I don't know from personal experience, but my LFS guy (who is awesome and has only given me incorrect info once in two years- and he made that right) told me that mollies, platys, and swordtails will all cross.
Mollies, platies and swords will not cross, sorry. Just because they are livebearers does not mean that they are able to interbreed. Mollies and swords are from different genus and that means that they are unable to interbreed. I have heard of molly/guppy combinations but never even seen a picture let alone a live specimen.
I'm going to try some molly/guppy variations in about a month or so. I have some black lyretail molly fry that are around 1.5 months old now and and are almost half grown. I'm not going to let the females breed until I try one or two with a male guppy. The others will be bred to a molly. What should I call them? Gullies? Muppies? I'm not sure ! I would definately post pics but I'm sure the dominent parent in genes will win the battle over what they look like.