I know this sounds strange. I'd get the yorkie a month prior and set the yorkie as alpha. The main thing is the pit bull has to be walked 45-minute per day to expend his energy. It really shouldn't be a problem as long the humans take on the alpha position. My son's former football coach had two pit bulls that were very obedient because the coach showed them who was boss. They would even jump into the back of the truck when told. I think they thought they were humans. They were cute because whenever someone would come around with food they were right there picking up the crumbs.
I have a cockatoo that's timid. I established her as alpha over our big dog when she was a puppy. The bigger dog wants to play but the cockatoo puts her in her place. She'll even climb off her perch and walk across the room, both dogs stay away from her.
Dogs sense fear and will take advantage of it. Again, I watch Cesar Milan (the Dog Whisperer) for a lot of common sense application to my pack (including my kids
).