I found a very fast and practical answer to getting the puppy rig complete without the need for all the technical stress of the adult rig. I simply went through the same method as I did to attach the new meshes. I placed the puppy mesh over the rig and copied the weights over from the adult mesh. It wasn’t perfect, but as a result, I was able to simply make brief adjustments and create several necessary joints such as redoing the tail and it worked fine.
Jess tested the rig and for the length of time the puppy is onscreen, it works wonders, hence I was able to cut a weeks worth of work making adjustments.
For the blendshapes, I just recreated the same ones as I did for the adult rig, but with more emphasis on the nose given its close composition to the camera.