I have 5 girls...none related and from all over the USA. Kissy was my first and she was an only child for 5 years before i got Angel. they had 3 major fights but i separated them for 15 minutes in separate rooms and after the 3rd time, they NEVER had another fight to this day. I have added #3, #4 and #5 and never had another issue with any of them. Mine range from 10 years - 3 years old and from 3 lbs. - 6.5 lbs.
Good luck! I hope you find the perfect little baby. I agree, the sooner, the better so they will grow up together since your first one is still young. She will love having some company her own size. I think yorkies do best when they have other fursiblings to play with. They do not like to be left alone!
p.s. All of mine except Kissy were brought into the pack after they were grown. Kissy is the only one that i raised from a puppy so don't think you have to start with another puppy unless you just want to.
The yorkie rescues are full of purebred yorkies that are looking for forever homes.