We have a checkered madtom (used to have 2 but one got eaten). He was about 2 inches when we got him now he's about 3 1/2 and he'll eat pretty much anything. They scavenge on the bottom just like most catfish.
Right now he's spoiled and gets handfed carnivore pellets. He's taken over a piece of clay pot we have in there and he swims in circles and wiggles when we come near the tank, waiting for carnivore pellets.
They also like live foods (assuming they can catch it - he'll ambush guppy fry) such as fish, worms, and small crustaceans, and many frozen foods or dry foods such as krill, bloodworms and that type of stuff, though it's up to the individual fish what they like to eat best.
The main criteria for a madtom is that it fits in their mouth, other than that they don't seem to be very picky, though I don't know if that varies by species or not. I had a tadpole madtom as a kid and he was pretty much the same way.