If you are setting up a paludarium/vivarium and it's not specifically for fish, I'd strongly suggest going with feeder guppies.
No clue if this is what you've already tried, or why you had problems with them, so...
Aside from that, they are pretty small fish that don't need much room to live in, and they tend to do pretty well in a wide variety of water conditions. Plus, their fecundity and live-bearing, when combined with even a meager bit of plants or shelter makes it likely that there will usually be some juveniles/babies hanging around.
As to killies, I've only kept the Pseudepiplatys annulatus, but it was pretty undemanding and entertaining. very active and interactive, and usually a top-dweller.