[yorkielady06]

Being an only pup is not an issue if the breeder works with the puppy. Some singletons do not socialize enough if weaned from mom and not allowed near her or other dogs. But health wise it makes no difference if the pup was a singleton or not, it depends on the parents health.
I hope the breeder has done the basic screening of the parents and has vet checked this puppy.
Parents should have at the minimum:
Bile acid tests
Hips and knees xray'd
Regular vet visits
Puppy at minimum should have:
Stool test
Vaccination and record
1 year Health guarantee
1 week supply of puppy food that the puppy is eating on its own.

And the pup should stay with the breeder for 12 weeks. Some yorkies teeth are not even all the way in by 8 weeks. During this time the puppy learns social skills and bite inhibition from its mother and other dogs. Puppies that leave at 8 weeks sometimes can have biting issues and can bark more/be more timid.
Has the breeder given you information about the puppies social skills and if she is dominant or a more submissive dog.. That is very important when adding a dog to a family that already has one. Is your yorkiepoo alpha? 2 alphas do not mix.
Has your breeder given you homework. Things to study and learn, things to purchase prior to coming to get your little one.
Just some things to think about.