I don't know very much about the actual service dog part of your situation. But I do have experience with bringing home another yorkie! Copper was just over a year old when we brought Rocky home, They did wonderful together. He was almost 2 when I brough Maddie home. It took him a little longer to warm up, but they enjoy eachother's company very much. It shouldn't be to hard to find a yorkie that is arround 7lbs.
However if your considering different breeds as well I would like to recomend the Coton De Tulear they are a bit sturdier than a yorkie. But they have a wonderful calm sweet temperment and don't shed. They love their people too like yorkies
