over on this side of The Pond, "20H" is a 20 gallon high. its actually a pretty common abbreviation in catalogs, LFS, etc (for the States anyways...)
its irresponsible to mix Endler's Livebearers and Guppies if FEMALES are present b/c resulting fry cannot be conclusively identified as purebreds or hybrids. if you have ONLY MALES, then sure. mix 'em.
any fry that are potentially Endler/Guppy hybrids should never be recirculated back into the population of Endler's. i personally think they should be destroyed. but they should definitely not be mixed back with Endler's because they'll dilute the species and result in further predation on natural population. what then happens is the problem of Lake Cichlids in Africa--there are more hybrids in people's aquariums than pure species in the lakes. continued capturing from the lakes to supply pure specimens for the aquarium has put many species on the endangered list. look, its just Bad Stuff, ok?
since Endler's are essentially just a subspecies of Guppy, pretty much all the same requirements and compatibility issues apply.
--EDIT--
what happened to all your other fish, metfan? i could've sworn you had some in there already...