Hi, Nick16,
just wanted to point out that Endlers really are a type of guppy, which is why they can interbreed interchangeably.
It's apparently hard to say how many pure tankbred Endler strains are actually commonly sold in petstores, at least around here, as this has occurred to such a degree.
Mine are a definite cross, the result of a lucky 'feeder guppy' buy - I personally like them better than Fancy guppies, in part because they are so much smaller.
But I did find it necessary to split up into different tanks my cross-bred males, which would otherwise harass the heck out of each other, which I hope isn't likely in Endlers generally.
And the more pugnacious ones tended to, at least initially, persistently display to other, bigger fish, who generally politely ignored them, which might not be the case if this occurred with shy and tiny fish such as those you're considering.
I have heard that Endlers don't tend to behave quite like that, but thought I'd better mention just in case.
If you are seriously considering getting some to keep in with what I've heard are easily intimidated Boraras brigittae, perhaps some of the many Endler keepers on the forum will by then have commented on their compatability.