Perhaps you should wait until your Yorkie gets over her fear of dogs. If she is afraid of dogs in a dog park how threatened will she feel when there is a large dog in her home.
http://www.yorkietalk.com/forums/4404027-post1.html I really think it would be wise and wait until things are more stabilized with your Yorkie. As Kristen stated the Dutch Shepard has a high prey drive and you state your Yorkie's attitude caused other dogs at the park to react aggressively.