It seems like you have already been given many answers. Do you think the responses will change if you ask the question over, or have you just not gotten an answer you are willing to accept yet? When you are breeding purebred dogs, you KNOW what specific issue plague the breed, what testing to do to eliminate prior to breeding them, and which are possible to be passed on genetically. When you cross breed dogs, you have no way of knowing how the gene pools from each breed will pair up in the offspring and what kind of problems may arise as a result.
(One example. Chis have eyes that protrude more than Yorkies and a very short coat. Yorkies have a coat that is longer. A possible result of crossing the two would be a pup with more occular protrusion, and a Yorkie like coat--including around the eyes, increasing hte chances of corneal ulcers.
Another example: Both breeds are prone to open fontanela. If that trait is passed on by BOTH parents, are you aware of how serious the issue could be for the pups?)
There are LOTS of reasons you could be given for why 'it's so bad.'
If you are not a BYB (and let's face it, your actions fit the description) then you have likely thoroughly researched and determined how best to handle the following situations and issues:
What risks have you determined will be heightened due to breeding a dog with a larger cranial structure to one with a more narrow birth canal?
What SPECIFIC medical test have been performed on dogs prior to breeding? (this goes further than just 'the vet says they are healthy')
Given that every breed is at risk for various breed specific ailments, which have you determined could possibly arise in the offspring as a result of combining the two gene pools?
Furthermore, the fact that you are stating they will go to their new homes at 9 weeks speaks volumes. Reputable breeders to not release toy breed pups until they are around 12 weeks. Are you even aware of WHY that is? If you are not a BYB, as you claim, then I'm sure you are...therefor, what justification do you have for releasing them so young and how to you plan to advise the new owners to compensate for that?