So we went to look at Amanda (yorkie-mix or silkie?) again on Saturday at the humane society. She perked up right away when she saw the four of us and never once growled at Addie and Mia. We took her home and she fits in like part of the family. There's been no fighting whatsoever. I'm simply amazed!
Amanda (now Dany) is 7 years old, possibly came from a puppy mill or backyard breeder. She is underweight, but we're working on that. She seems uncertain about grass and snow, like she's not sure she should walk on it and she is not housetrained in the least. We're working on that too.
Dany is an absolute doll, just like Addie and Mia. Addie (who I was most worried about) doesn't mind at all that there's someone else living at home. I'm thrilled to add her to the family!