First of all, 6lbs is TINY! Do you even realize how incredibly TINY that is?! My sister's yorkie is 6 lbs and she is basically the same exact size as my 3lb yorkies but she is just a little more hearty. She is still TEENY TINY and you can tote her anywhere and everywhere you could possibly want to tote her.
Second of all, both of my yorkies were PORKERS when I brought them home at 8 weeks. The charts said they would be 6-8lbs full grown. They aren't...they are 3lbs. So you really honestly cannot trust the charts.
Thirdly, there is nothing wrong with pit bulls but you absolutely DO NOT need a 3lb yorkie in the same house with a dog that size! All your pit bull would have to do is lay a paw on a 3lb yorkie and it would be a GONER! You obviously just have NO IDEA how tiny and fragile and 3lb dog is.
Here are pics of my sister's 6lber and my 3lbers.
In this pic, my yorkie in the middle is still a young puppy and that is why she looks so much smaller. The yorkie on the far left is my 3lber and the yorkie on the far right is 6lbs. They look almost exactly the same size to me.
Here the 6lber and the 3lbers look like they are the exact same size.