i have one at round 18ish, he's with a lap, tsn and GG, they are all smaller than him but i never had probs... its usually the gg who becomes aggressive
I don't have a marbled catfish but a 12 inch brown bullhead. They don't like to be out competed for food and they don't like other fish bumping into them and snatching the food the smelled in front of them.
You will notice your catfish bite your other fish if it gets too close. They will continue to bite the fish until it has moved. Give your catfish quite a bit of food that will sink quickly. That way it has it's own area to feed. Feed the other fish away from the catfish's area.
You may also need to leave a dimmer light on. Dim enough for some darkness but light enough for ATF, etc... to see clearly.