Having had Endlers in with a betta in the past, I would say you have about a 25% chance of trouble free success, about 50% chance of success but with problems, and a 25% chance of failure. This is because it will have an awful lot to do with the fish's temperament, from both the betta AND the Endlers. It seems hard to fathom that Endler's have different attitudes but it is true. You'll have some that are laid back and trouble no-one, you'll have some that love nipping fins, you'll have some that want to pick a fight with each other, and some that harrass another breed hoping for the chance to hybridise.....lol Then of course you have to factor in the bettas, will he tolerate other fish in to his territory, will he see them as small enough to eat, or will he just turn a blind eye? What happens if they nip him, would he then snap back etc etc etc
There are so many variables, but obviously I want to give some helpful advice on how to make this work. If you don't already, get the betta a cave type object, something he can 'retreat' to if he wants some peace. Add in dense plants (the Endlers love these anyways IME) for them to retreat to if they feel threatened. On the day of adding the Endlers, remove the betta for an hour or two when you add the Endlers, then put him back in once the Endlers have figured out the new tank. Then watch closely for a while to see how they respond to each other. You will likely see the betta flare a few times, as he is just going to state his dominance. If he attacks, remove the Endlers. He might just ignore them, in which case watch the Endlers because they can be nippy and get over confident and gang up on the betta and nip his fins to shreds over time.
There is a great chance of this working, but you may have teething problems as they settle down. Endlers are better in small groups, with more females than males, an all males group can have problems with fighting when they decide it's time for a family.
All this is from my experience, so could be wrong, but I hope it's helpful.