Congrats on making the decision to add a new Yorkie to your family! Even better that you are considering adopting!
Adopted dogs may be older, but they are usually kept, monitored, and trained in foster care so your chances of "knowing what you are getting" are actually best with a reputable rescue as opposed to a puppy from a breeder.
All of the rescues I have adopted from or worked with have all required an application, home visit, spay/neuter contract (if the dog was not already fixed) and a contract stating that the dog is to be returned to the rescue if you no longer can keep or care for it.
I suggest asking the rescue about the particular dog you are interested in, and if you can meet in a park to allow your dogs to meet to judge how they get along on neutral territory.
Once you decide on a dog, here is some advice on introducing them in such a way that increases successful integration:
LDT House Training
Adoption is a wonderful option! All of my dogs are rescues!!!!
