I agree with Betta123, instead of Guppies do Endlers.
I actually disagree. Endlers aren't weighed down by heavy fins that guppies have, and if anything, are more energetic. I would avoid them.
One livebearer that could do perfectly well in that space is Heterandria Formosa, the Least Killie (which isn't actually a killie). It's the smallest of all livebearers, and what it lacks in color it more than makes up in personality. The males are always having sparring matches (never saw any injuries), the females are always actively ignoring the males, and the males are always pouting about the females. Extremely entertaining, haha. They also don't reproduce out of control, might give birth every other day, but it's generally only 1 fry at a time, every 1-5 days.
I had a breeding colony of over 20 once in a 5, and 3 males in a 3/4 gallon bowl. Honestly, there was no difference in behavior, except the males desperately wanted females (and decided the smallest male was female enough). You could probably have 4-6 in a 2 gallon bowl, if it was well planted, filtered, and had regular water changes. Heater is optional- Het formosas can handle 40-85 degree water. 60-75 degrees seems to be the best range however.
Some of the best fish I ever had, even if they weren't as colorful as guppies.