hello...I got my yorkie baby EXTREMELY early at 8 weeks old. ( I didnt know any better at the time) Anyway, I also have a Westie almost 4 years old and my yorkie just runs all over him! She is relentless and he is so too tolerant of her so I occasionally have to step in. What I do is get on the floor beside my westie and give my yorkie a low growl (like Mama would have done)and this helps a lot! She gets the message immediately and my westie gets a well deserved break.
Also, if she bites me too hard even tho playing then i just give a high pitched yelp and she stops immediately. I find this a pain free discipline and it works for me. Hope this helps!