Im assuming the above posters know nothing about the subject or otherwise would have refrained from making those comments. Yes, you can cross the two species. If you cross a male molly with a female guppy you will kill your guppy. If you cross a female molly with a male guppy you MIGHT get fry and some MIGHT survive and they MIGHT not have the genetic defects common in such a cross but you won't EVER be able to produce viable offspring from the crosses themselves as they are essentially mules, and those that have been successfully bred have never survived to produce a viable breeding population which is why you don't see Gollies in the stores everywhere.
So good luck with this if you want to take it on, but I'm going to stick with my initial opinion that it is not worth your time. If you're looking to create interesting hybrids, try some africans or cross different coloured guppies or even do endler X guppy crosses although you will incur the wrath of the livebearer purists for doing so.
As for my tanks.. they pay for themselves and are damn near turning a profit and they keep me busy and happy. I'd say they're worth my time thanks.