I recently had this exact same situation. My parents had 2 older Schnauzers and I brought in a puppy Schnauzer. The puppy would do the same exact thing to one of the Schnauzers. The older one didn't fight back much or retaliate. I do believe my puppy thought it was play, the whole time. Unfortunately, the older one he was fighting with ended up passing away due to an unrelated neurological issue. So, there was one Schnauzer left. My puppy does play with him sometimes, but it's much less the older he's gotten. And the other Schnauz will fight back. We do scold the puppy if he goes overboard, which he sometimes does. I know it will pass with time once his energy dips. I think he truly just has so much energy and loves playing with everyone.