peckoltia sp. and hypanstrus sp. all stay fairly small. They like their water a little warmer; I keep mine at 82-84. they also won't eat algae and require meaty foods. Bristlsnose varieties are all good, too. Albinos and calicos look nice and they also eat algae very well.
some of the smaller panaque sp. (L104, 204, LDA01) stay smaller, but eat wood and can make a mess.
check planetcatfish in the catelog. I'm pretty sure you can search by family and also by size.
Lemon spotted are pretty cool and very different. Adds a nice pizazz to the tank.... zebras are nice too.... both are pretty expensive as far as freshwater fish go though.
oh yeah, most of the nicer ones aren't cheap. L200, which is probably the lemon spotted plec DD is talking about, usually goes for $35-40 here. Zebras are in the $200 range.
A nice cheap dwarf fish is the albino bristlenose. They're $3-4 if you can find a local breeder, or $6-10 in stores. Clown plecos (L104) go for $5-10, and rubberlips are in the $5 range.