Sigh... I had pure endler's once upon a time.. but oh no.. I had to find out if it was possible to crossbreed with guppies. Oh yeah it is... the resultant fry are fertile, less agressive, and show a mixture of traits.
Oh, and check with your local city. I originially got endler's livebearers as a free biological control for mosquito's. The city found them easy to breed, and gave them out free to anyone with standing water. One of the biggest pros of the city using endlers was that no endler could survive the winter.. so little ecological impact past one season.
I understand the city switched to common guppies since.. darn it!