Rocky was better on the inside but if Cooper landed his body shot/right uppercut combo he would stop the smaller Rocky, having said this Charles and Walcott would have done far better v Cooper than they did v Rock, different level altogether, all Cooper has over Rock is size, which could be a factor as he used to come out swinging, he would be wide open though and Rock would catch him at will, which I feel would be too much for the inferior Cooper.
I would bet heavily on Marciano within 6, and that's even assuming the best Cooper shows up, which is rare
Cooper and Marciano!! Bang .. Crack... Smash. Rocky has to climb up off the ground to stop Bert in 7.
The perfect version of Cooper could certainly beat him. But we rarely, if ever, saw that. So, it's just not in the odds.