Hi Alison,
It was very nice of you to have taken the stray dog in your family.
I have a couple of concerns though:
1) How can you prevent the stray getting pregnant while your yorkie is not neutered? I understand that both dogs are not ready for a surgery at this point so please be very cautious about that.
2) I believe you should make your yorkie as a top dog. You can manipulate their ranks as the pack leader. It is common that people let rescued dogs come too far by being sympathetic to them and being too soft. It is actually better for the rescued dogs to become aware of the house rules and boundaries as well as where they stand in the pack as early and as clear as possible.
I foster rescued dogs and I place them at the bottom of our pack (our 3 dogs are above them regardless of the size or the personality of the rescued dogs). I strongly believe that your yorkie should not give up any attention, care and love he deserves by the fact you brought in a new dog. The new dog was starting from no attention, care and love so she would appreciate being loved and cared even if her ranking is below the yorkie.
Your stray terrier is a juvenile or teenager in dog's age. She will soon try to expand the boundaries and see how far she can go. I see she is already challenging it by being bully. You are inadvertently reinforcing her behavior by feeding her first, etc. I would urge you to start a firm and consistant obedience training with her.
Good luck
