I agree that i is not so much the breed but the size, we are debating this same issue in our home right now(the SO wants his own baby).
Copper plays well with my moms mastif pit mix but through the puppy stages we had to have alot of very controlled interactions he is 4 lbs and she is 80 lbs we both got the two at the same time and they have really grown together it is alot of work and still we never leave the two of them unattended. I think that it would be fine to get the doberman it everyone in your home is willing to take on the extra work of making sure that your yorkie is safe. I have to say that it is so neat watching such a big dog with such a little dog
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