The OP already defined what was too "big" as a Jaguar catfish was "too big" Theres not many jaguar cats hitting over 8'' mark anyway
Pictus are way much more graceful than a raphael when maneuvering around the tank - They're way more streamlined so not put that in the equation though.
36''x18'' is not comfortable for a 8'' raphael catfish to maneuver around the tank. I know from my experience as downsized myself and those guys are very restricted right now as once I move, I'm getting those guys a bigger footprint tank/indoor pond whatever I can do in the new place. Their current tank is 4' with 12''-16'' at the widest point its flatback hex- with what I've seen 18'' will not cut it.
Plus to add: he is planning a catfish tank with multiple sets of cats- they need PLENTY of hiding spaces so therefore decor is a must such as driftwoods, rocks , plants whatnot OP decides to use That will severely cut the open swimming space for a big raphael.
Imagine a clumsy knight in a very very ancient mail armor.