You could look at the livebearer section of the parent forum, Tropical Fish Keeping for some ideas.. http://www.tropicalfishkeeping.com/livebearers/
I keep both Platies and Guppies, but not in the same tank, but I do plan to have Platy/Guppy show tank in my 100 gal when the fry get older..
I understand that Guppies and Endlers are quite similar in genus, and will cross breed making guppy/endler hybrids.. I have also read of guppies and molly's being bred together as well..
Platy's are of a different genus, so wont cross breed with guppies..
Platy's and Swordtails are of the same genus and will cross breed..
This probably covers the more "common" livebearers and are the ones that I have kept at one time or another over the years.. There are many other less common or "exotic" if you like, livebearers, that are quite beautiful fish as well..