In a 55 gallon, you won't have much choice for predatory fish of any kind. Catfish that are big enough to eat other fish (or have the inclination to), get WAY too big for a tank that size. Channel cats, shark catfish, etc. get anywhere from 1-10 FEET in length (depending on breed and circumstance...I've caught channels in my local lake that were 5 feet before....yep, the same ones that are 3" at the lfs...).
You can have aggressive fish without necessarily having fish-eaters. I got on a "super-mean" kick there for a couple of years, and all I wound up with was an empty wallet an dead fish, since I couldn't provide adequate tank space to keep anything that was truly predatory. I have a 55 as well, btw.
I still like the aggressives, I have just decided to wait until I can give them the kind of home they deserve, at least 200 gallons of it.
You might give a try to a NA native, like a bluegill (only one or two for 55 gallons, though). They're meaner than snot and will happily eat anything that will fit in their mouth.... Small perch species like that are really pretty and have an attitude to match...
Also, I'm not a big fan of ropefish...they like to escape. Plus, anything predatory with a mouth larger than a quarter would make a quick meal out of one....they do resemble a worm, after all...
One more quick note: I wouldn't feed exclusively feeders to anything unless that's all they will take. If you do, only feed them weekly, because the ammonia will shoot up like you wouldn't believe and be uncontrollable.... Try to get the fish accustomed to pellets, as well, or you'll not have your fish for very long....