I just got a new yorkie puppy as well about 6 weeks ago, at first Bella didn't want to have anything to do with him at all...she would growl and snap at him.
...Than I had my hubby start taking care of Harley so I could give Bella all of my attention, within a few weeks she was playing with him, no more growling and snapping at him...you have to pay the same amount of attention to them if not more than the new pup so they know you are not replacing them with the new pup...it worked for us Bella and Harley play well together now. I hope this helps and good luck.