I'm not sure if using your old trainer without ailerons for practice would help. I think the key to rudder use in a 4 channel setup (throttle, elevator, rudder, and aileron) is to make aileron turns cleaner. Using the rudder on your trainer as you've mentioned would get you use to using the rudder on the left stick (assuming Mode 2). However, I don't think it would help you co-ordinating your turns.

I'm by no means and ace flier, but I remember being told by my instructor when I learned to fly to move you thumbs in unison. At this point it's somewhat of a secong nature, though I must admit there are times when I catch my self using ailerons only for turns.

Don't forgot some day you might progress to a model that won't turn without rudder even with ailerons! The ailerons will just bank the plane due to adverse yaw!