I just wanted to say that you are a saint for taking in a yorkie in need. The world needs more people like you
We just took in an older foster (the vet says anywhere between 10-14 years old), and she has adjusted well with my two. We kept her separated from my two while we were not able to watch them, and slowly integrated them. She now stays in an exercise pen in the room that they hang out in and does well. She sleeps while they play and just kinda walks around when she's awake. I'm used to puppies and younger dogs, so she is a pretty easy dog to take care of.
Good luck, and keep us updated on how she does. I'm sure she will be a great addition to your family!