There are a couple naive species that might work. I have a brown bullhead and not only is he predatory but also very interactive, will eat right out of my hand. Browns are a bit large for a 55g but the spotted bullhead
Ameiurus serracanthus only reaches about 8-10" and could work. They are also attractively marked, I've seen them at the LFS under the name marbled bullhead.
The margined madtom (
Noturus insignis) is another option, they reach about 6".
This site had them for $10.00 each.
http://jonahsaquarium.com/JonahSite/fishlist.htm
The stonecat (Noturus flavus) is another good one, they reach about 10". I had a small one manage to get into one of those livebearer fry nets and eat every single molly fry I was rearing within a couple hours, predatory without a doubt.
If you want something tropical look into the Asian upsidedown catfish (
Mystus leucophasis). They can reach about 12" but males remain a bit smaller than females. They hide a lot but are pretty far out looking.
Here is a crummy picture of mine.