I just want to say, I believe it is how you raise the other dog with your yorkie together.
My boyfriend has a akita chow chow mixed dog (he's around 120lbs) they get along fine. When lexi first saw him she freaked out! literally freaked out and peed herself but then we just let them be, now they are best friends. When we take both of them to the dog park and other dogs bother my lil Lexi, he looks out for her and protects her. It's cute.
People always laugh when me or my boyfriend walk the dogs together because there is such a huge size difference.
Sometimes we leave them alone together and they are fine. She stays in her corner he just chills there. when we come home they are both sitting down wiating patiently for us to come inside
I believe a dog will behave good or bad depending on how it was raised by the handler.
Good Luck!