I'd suggest a single fish that's pretty good at hunting rather than a school, mostly because that way you'll end up with just a few (really fast) fry surviving, which is nice because endlers aren't terribly long-lived. A dwarf gourami could work, or a female or plakat betta. Both species can be a bit iffy with endlers, though, they often take all that flashing and long-tailed-ness as a territorial challenge. Otherwise, you could try a smallish (6 or so) school of tetras and a bunch of good hiding spots.
Angelfish will eat adult endlers, too. Not your best choice.
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My current project, a 65 gallon aquarium stocked with vernal pool fauna.