Just ask Cesar Millan, the Dog Whisperer.
But the Pitt Bull MUST MUST MUST be calm and submissive to everyone, including the yorkies. Pit Bulls are dogs bred to fight, and they have that tendency, but can be calm submissive dogs if they are taught to be. The Dog Whisperer is on the National Geographic channel. I would watch that and you will get some good ideas of how to train him right.
~Melody