I'm basically in the same situation. I have a boy who's two years old, 4 pounds and is nuetered and very attached to me. I just got a baby girl, she is now, almost 15 weeks old and under 2 pounds.
My boy, Jaden, has never been aggresive in the slightest. Doesn't bite, nip, or even really growl or snarl. He's as sweet as pie! When I brought my little girl home, the first day I had Jaden in my arms and she came up to pull on the blanket he was sitting on and he started growling at her. I told him no but he was upset...the next day I decided to let her down with him and she went to go into his bed and he got very upset and snarled at her.
Since then I've kept them seperated unless I am there to watch them. I started off just holding both of them in my arms for a few minutes everyday. Then, if we went someplace new that neither of them felt was "their" territory I'd let them down and Jaden didn't seem as aggresive towards her. He still has his issues, especially around his bed, he doesn't like her there at all. He's not used to being with another dog and my vet said these things can just take time, even a few months, he said some dogs adapt faster than others. For now, I keep them in their room with a baby gate dividing them so they can still see and smell each other through the gate but I know no one is going to be getting hurt if my back is turned.
Be sure to give your older yorkie plenty of attention. Jaden will get very jealous of Jewelina if I do not give him TONS of attention! Jaden always gets the first hello pat, gets the treats first, groomed first everything. My little one does complain about this sometimes but I think it is mainly because she likes Jaden so much and wants to be with him and doing the same things that he is doing and not that she feels "second best" or anything like that.
I am hoping in time, perhaps once Jewelina calms down a bit, (she is a typical puppy and wants to play 24/7 and Jaden is very layed back) Jaden will learn to like her more. I don't worry about her at all because she already loves him...I'm just waiting for him to come around and do something more than just "tolerate" her.