Guppies? Psh.
Do puffers. I've got a 10 gallon, and I currenly have 3 blue eye rainbows, 3 blue rams, 2 yoyo loaches, 2 kuhli loaches, and an asellus puffer.
The asellus puffer is the cutest darned thing I've ever seen. He's really peaceful, never nips on any fins (my yoyo picks on him sometimes, though) and you can keep several of them together. Best of all, they're NOT brackish. They're completely freshwater.
I think they're also known as the Brazilian Freshwater Puffer. Anyways, here's some pictures:
<IMG SRC="http://www.psyanide.com/pallokala/gfx/kuva1.jpg">
<IMG SRC="http://tekipaki.jp/~puffer/puffer/colomesus/asellus/02.jpg">
<IMG SRC="http://www.remix-net.co.jp/Nettaigyo/photo2001/list_tetraodon02/colomesus-asellus1.JPG">