Well, there's 2500 or so catfish species, around a third (800-1000) are "Loricariidae", which is where the Plecos etc reside in the family tree. That leaves 1500-1700 species that aren't Pleco's.
Of course, several hundred of those disqualifies themselves on other grounds (too large for a 55g tank would be the first and most obvous one, followed by too small, will end up eaten).
But there's still a huge number of fish left - unfortunately, it's hard to sit here and recommend something out of that - because it's a personal taste thing that decides what you like. It would also help to know the temperature of the tank, for example.
Something in the
Doradidae family perhaps? There are some huge ones in there which wouldn't work, but the smaller members are nice - although some, like the striped Raphael
tend to hide quite a bit.
Fast swimmers may also be OK, like
- even if the JD is unhappy with them, provided there are sufficient hiding-places, they should be fine, as long as they are big enough not to be swallowed.
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Mats