In general, there are certain livebearer species that are able to crossbreed with another and there are lots of livebearer species that are not able to cross with another. Not particular that it concerns another genus but more because of the fact that their sexual organs should be compatible. Otherwise, no natural mating is possible.
Within the Poecilia family, guppies and endlers can crossbreed and have healthy and fertile offspring.
Very close related species won't bother to hesitate to mate with another, like platy x swordtail, endler x guppy or limia x molly. But when we're dealing with livebearer species that are less related but their sexual organs are compatible with another, they can cross as well. The fertility of such crosses can be fertile or infertile. But such less related species kept together in one tank with the presence of both genders of both species, the first choice will be that they'll mate with a specimen of the other sex of their own species.
A lot of socalled endler strains in the commercial world are already hybrid strains between endler x guppy. With endler hybrids it's always a guess wether they tend to have more endler or guppy traits. Especially, the really mixed ones. So, you can not always say that a pure endler will be stronger than an endler hybrid. Or the other way around.
Endlers and endler hybrids are in general stronger than fancy guppies with large fins. Short finned fancy guppies are also quite hardy.
That's indeed the best thing to do...
Most fish farms that breed endlers will focus on endler hybrids. This is simply because they can create more colorful specimens by working with hybrids. Such hybrids are commercially more attractive and are worldwide easier to sell.