I have tried this with the R23 Character Definition, and the proportions were very problematic. What I did was that I set all the Advance Biped Rig’s controllers to FK. Instead of extracting the joints, what I then did was I extracted the controllers using Character Definition (arranged them to their corresponding parts). Then, I linked my Mixamo Rig to my Advanced Biped.
Through this, my Mixamo Rig was able to control my Advanced Biped. But, the proportion, as I said, was problematic. I was hoping that this problem was solved by the new updated on Character Definition. Was it fixed?