OOoo! Nice tank. You might not have this in your area but I've seen it at the LFS and I've done research on it, and they look really nifty.
Asian stone catfish/ anchor catfish: bottom dwellers that don't need much space(only 3 gallons). They eat catfish pellets and protein rich foods. After they get used to the tank, they will start roaming the front. Also, looks like you have plenty of space on the bottom as well as nooks and crannies they can hang out in.
otocinclus catfish: You've probably heard of them, tiny and cute clean up crew fish, and can live in a variety of conditions. They do need a group though, so you should take that into consideration
Shrimp: Really any type of shrimp - amanos, cherries, crystals, etc.
Chili rasboras seem like an excellent schooling fish for this small tank
Endler's livebearers: Really small, cute, and come in fancy colours. They will also breed tons of babies.
These are just suggestions, but you should check that they fit in your water parameters that your tank already has.
