I've no experience with this type of size difference, but if it was me, I would have the GS on a leash and right beside me anytime they were together. A 13wk old puppy has no idea that it can be hurt and most Yorkies are fearless at this age. You need to be sure that your puppy doesn't bully your GS, as puppies can have a tendency to do
and you need to be very sure that your GS doesn't accidently hurt your Yorkie with his play.
All that said, I would love to see pictures of both your babies!
