7 tiger barbs would make up about 21" worth of grown fish, really good for the tank. You can also get different colors, according to others on the forums here, as the presence and pattern of color, rather than the specific color itself, is what triggers the schooling instinct.
So this leaves you with about 8" of fish left.
If you want something that will help keep the tank clean, go for a couple of otos (otocynclus catfish). They're really small (and a bit difficult to
acclimate, but I think they're worth the trouble). If not, don't worry about it.
Another set of good helpers would be corydora catfish. Cories need to be kept in groups, but you could get three dwarf cories to top the tank off.
If you're looking for something other than bottom feeders, avoid anything with long fins or that is slow moving. Most other things should work well.