Well, I would first suggest that you find a more appropriate play mate for your puppy ... another GSD puppy in the area? (I am NOT advising you to get another one!)
Or another "big" dog puppy in the area? GSDs are rough when they play, and while the min schnauzer is putting your pup in his place, the spaniel probably never will. Some dogs will "correct" and others will cower.
And of course he returns to the spaniel, in his world, it's fun. You won't get him to "understand" it's wrong, he's a puppy. It's like trying to explain to a 2 year old child that you're going to the toy store but you can't touch or buy anything!
He needs an appropriate playmate to be able to play like a puppy ... hence the suggestion of another "big dog" puppy, or another adult dog that LIKES puppies and will properly correct the puppy when he's too rough. You need to make sure that the adult dog is correcting PROPERLY and not being a bully to your puppy.
Marion’s Zoo-Kyleigh, Raylan-cat, Echo-TAG,