Now I can see him better he looks closer to 4". There is no immediate problem with him harming my pickerel. My pickerel could eat him and is a little over twice his size, but the madtom/bullhead stays hidden most of the time.I can see the tail very clear so it's 100% yellow bullhead. These yellows are very aggressive.
As you said he seems to be too big for a madtom. Very few madtoms that reached 4 1/2 to 5". Better get rid of him, since yellow are one of fast growing fishes and he don't have any problems to put a pickerel away.