I flew with rudder, elevator and throttle for eight years before my first 4 channel aileron plane. When I made the transition to ailerons it was absolutely effortless. I would not hesitate to train someone with rudder and no ailerons, and I have in fact done exactly that. The students had no trouble flying with ailerons later. There is no downside. I flew my three channel planes in wind that other pilots didn't fly it. Put the rudder on the aileron channel, and yes, you have to have dihedral so the rudder will induce a roll.

Jim