you could do a nice Betta tank, or a nano tank with some Dario dario. Endlers livebearers are really small. If you can afford a larger tank, it will be easier to keep stable. And of course, you can stock more fish.
Have a look on liveaquaria.com at the different species and their minimum requirements to get a better idea of what size tank you need. You can always ask people on here for guidance.