I was browsing around on the Aquarium Wiki on fish which could go in a 5 gallon tank. It says 4 males could go in a 5 gallon tank.
Would a male Betta get along with them in a 6.5 gallon tank?
Heres the webpage I was talking about.
If you do about 3-5 male endlers, lots of hiding places, lots of plants and a good filter running purigen or carbon, it'd be doable. The plants and the hiding places would help the betta have his own space, although it really depends on your betta. I'd do it with my SD because he's laid back and he's in a tank swarming with endlers, however I'd never try with Roommate's VT. Because it's a small space you have to go into this telling yourself it may not work and you may have to separate them.
No female endlers though, you need 2 females to a male and every 23 days they drop 10-20 fry. You'd soon be overwhelmed and your tank would suffer.
Honestly, I'd try RCS and Amano shrimp. They're colorful and are great additions to a tank. You could easily put 20 RCS and 2 Amanos in a 5 as long as there's moss and hiding spots and they'd barely put a dent in the tank's bioload. The Amanos won't breed, but you can easily get the RCS breeding like rabbits. I keep Super RCS and Amanos with my bettas and I love the way they look.