There's really no gravel that *should* go in there, per se, however, a darker (And more fine id definately recommended) one will better contrast the colors of most livebearers. This makes them, more or less, look pretty
Otherwise, like was said, guppies and platys are the best options since mollies and swords are a bit too large.
Platys are a bit faster, and (As far as my knowledge goes) will nip at algae somewhat more often.
Guppies are generally a bit fancier, but there are color differences between males and females.
Either way, it's presonal preference, and you should get no more than 5 of one or the other.
A planted species tank of Endlers would look amazing too. Otherwise, as mentioned, Guppies and/or Platies. Figure out your plan for fry first though, because I can guarantee you'll have some as long as you have females.