Sounds like he's just playing biting just like our Yorkies do.
I would just practice with
the two of them around each other, and teaching BOTH dogs boundaries and show them
what they allowed/not allowed to do. It's perfectly capable for a little dog and a big dog
to become great friends. As long as the GSD is not dog aggressive, or aggressive in general,
I think you will be fine. Show your Yorkie that when the GSD is around -- she gets a treat,
so she associates him with good and positive things. When the GSD is doing his
playful biting, gently take him away and say 'gentle' or any other word so he learns
what is acceptable and what is not. Basic training, if not done already, is always
a great step for both dogs to learn sit, stay, down, away, etc, so that if you think
things are getting out of hand, you can give them a command to obey.