On a more serious note, I would never MAKE one of my yorkies be my service dog, I too would be more inclined to get a trained dog who already knows his job.
I also think it would be very difficult to MAKE a yorkie be anything it doesn't want to be.
At this point my two are pampered, spoiled and rotten and I cannot see changing things on them now or ever. I believe I will just enjoy them and pray that I never need a service animal.JMO.