Thank you all for your input! All day the house has been peaceful, no fighting, no growling. The 2 Yorkies are like best friends. Let's see what happens tonight. As I said in my original post, I may be contributing to this behavior by favoring the puppy too much. When I first brought him home, the 2 older dogs used to give him a hard time, now he has turned the table on the other Yorkie. He doesn't even attempt to fight the Terrier. The Terrier has about a 10 lbs advantage and the Yorkie knows this! He's no dummy! Will post tonights results!
@juliealfies mum: My girl fights back. No blood has been shed. But I still don't want this behavior to continue. I always have to be careful when I break them up because one of them could easily bite me.
@gemy: I'm leaning more towards the crate. Right now he gets a very firm NO and then I put him on the floor. If he exibits the agression tonight then he'll spend the night in the crate. Thanks!