hybrids are always tricky, generally I've seen molly/guppy hybrids in live bearers. and If you plan on attempting such. my best advise is to breed male guppies to female swords. same ratio as if you where going to attempt breeding most LB's and 1/3 though I generally do a 1/5 ratio on males to females. But to pull this off you have to also understand that they may simply not breed, you will have a very high mortality rate, and a very high "mutant" rate. which i lump defects,internal and external, poor immune systems,sterility, ect. the females must be kept in optimal condition at all times. I "breed" guppies and mollies, that is to say I have a tank of guppies and a tank of mollies. I haven't personally tried to cross my two for those very reasons. there is also a very good chance at loseing many females this way due to fry death, septisemia type complications. But it indeed can be done and has been done for many years. to be honest I'm almost 100% the young are sterile as I've never heard of or seen fry from such X's produceing young of their own. But i certaintly could be wrong.
And there are also specially bred guppies that look like swordtails.. which are simply a genetic trait bred into them to have a swordtail look. the lower part of the fin is colored while the top half is clear. I haven't seen any guppies with true swordtails. but I'm not a show quality guppy breeder who could give a diffinitive answer to that.