I second the suggestion of Heterandria. In tiny tanks, having lots of very small fish is much more fun than just two or three platy-sized beasts. With a decent filter, a 10 gallon can hold a couple of dozen Heterandria, and you'd be a blessing to other livebearer fans, cranking out babies of these rather nice but rare livebearers to share with your friends!
Though called mosquitofish, they're actually perfectly peaceful, and not at all aggressive, as "real" mosquitofish are.
Cheers,
Neale