You should observe some things... this may or may not help you not really sure.
You say that the GSD plays rough and mouths on your smaller dog, and your dog yelps, correct?
So when your dog yelps, how does the GSD react.
We actually yelp to help train our puppies to not be too rough with us, and help with the bitey behavior.
Part of bite inhibition is that yelp means OW TOO HARD, so its actually teaching the both of them to play less rough. If the GSD stops immediately at the yelp, thats good. And also make sure youre dog doesnt seem bitter.
Most dogs forget it as quick as it happened.
I would keep them together assuming that what I said are facts, with some alone time. Spend time with each of them solo, them together, and make sure they both have calm, relaxing crate time.
Maybe work on relaxation training with the two of them in the same room to make sure that they can be calm and comfortable around each other.