Welcome to Yorkie Talk. There are many people here whose smaller Yorkies have been spayed and anesthetized for other procedures like dental cleanings.
Have pre-op bloodwork done and discuss the anesthesia protocol with the vet. It is good to look for a doctor who has experience with smaller dogs.
http://www.yorkietalk.com/forums/yor...-protocol.html
Breeding is risky with any size female Yorkie, and the risk increases greatly under 5 pounds.