The little girl will probably get no bigger, though she may "fill out" and bulk up a bit, meaning she will put on a little more weight. Two females are fine, as the new girl is still very young. Right now I have a four female pack of estrogen
, and everyone does fine. Please buy her, and bring her home. Take her to the vets, make sure she is healthy, has shots, spay, heartworm test, flea and tick protection. You will know quickly if she will fit into your family or not. Worst case scenario, you may need to rehome her or give her to a Yorkie rescue group for them to place. I think it was destiny that you found out about her, and even if by some chance she is not your "forever" girl, then you can still give her a safe, loving home, where she will be cared for and cherished. In the end, that's what we all want for her. My guess is that once she gets into your home and your heart, she won't go anywhere. Congrats on your new fur baby!