I am certainly not encouraging the breeding of any yorkies period. It is not for the faint of heart.
In my opinion
breeding yorkies of any size is risky because of their size and the fact that they are a toy breed. There is a higher percentage of c-sections in this breed for that reason. 2.5 lbs or 6 lbs......it's a risk. A 6 lb yorkie can have a 12 lb yorkie!
I give a lot of credit to the breeders who are able to do this because I could never accept that risk for one of my babies. But...if everyone were like me, there would be no more yorkie babies.