This shouldn't be as hard as it sounds since catfish are generally nocturnal.
Feed your other fish in the daytime, then come in at night when the other fish are sleeping and drop in a sinking catfish pellet. Your cat should be about the only one up, and even if he's not, the advantage goes to the one with the best nose!
Our cat can find a pellet within minutes of hitting a random spot in a 55 gallon aquarium with currents from a powerhead and Fluval return. It's like radar! Don't worry, he'll be able to outcompete the others if you make it on a catfish's terms.
Otherwise, get a more aggressive cat. Our's is a Raphael-well known for being the PERFECT community fish. They bother no one unless bothered, but EASILY capable of standing up to an aggressive Oscar.
Our's is often fed during the day when we see him up, or want to! It gets boring seeing him only every few weeks. The pellet goes in, and our barbs are all over it. Then the Raphael come is, grabs the pellet in his mouth like a dog with a frisbee, swishes his body through a few 90 degree arcs to discourage any pursuit with his barbs, then disappears into his cave with his trophy. NO ONE follows him in!