Everyone has given good suggestions so far

I thought I would throw a couple more ideas out there:
1 BN Pleco: 5 inches
4 X-ray Tetras: 8 inches
3 Albino Cories: 9 inches
Total: 22 inches
Remaining Room: 7-8 inches
With that 7-8 inches you could do one of the following:
2 Bolivian Rams
2 Pearl Gouramis
1 Angel
I wouldn't go with a school of barbs unless you take your tetras out of the tank. You don't really have room for a large school in there right now, and if you get just 2-3 of them, they will get very nippy.