I couldn’t find a way for the bones, sadly. I wish there were shape keys for armatures. What I did was make all the characters fit the same skeleton (hands in the exact same position, feet too, and the eyes and the mouth as well) This way It’s easy to change the face & body with the hundreds of shape keys the plugin has, and make unique characters.
Because they all have the same skeleton, I can transfer the mocap data super easy. If I want to change the proportions a bit, I scale the armature in pose mode and add those extra variations.
About the SP skin, there’s a tutorial I liked (https://texturing.xyz/pages/making-of-underworld-viktor-bill-nighy) but it ultimately didn’t work for me (because of this tutorial the skin shader looked insane!) The skin, now in SP they have tons of skin materials, and I apply them with masks to the body parts like this (https://substance3d.adobe.com/magazine/substance-source-micro-skins-body-parts/)
About the morphs, Transhuman4Blender has a base mesh that is genderless and has no ethnicity. Then we have hundreds of morphs to change every aspects of the face and the body. To make it a bit easier, I added five female base faces and five male that can be used as a starter point, and there are one female & one male body types, and then weight -, weight +, and muscular for each.
Besides all this you can add age and make the characters look really old. You can save it all in presets to keep each character and reload it any time.
So like this alone you can already make any character you dream of, but I got carried away, and I added a randomizer, so we can create background characters with unique features with the click of a button! You can also randomize the characters you have already saved to find new variations.
And then there’s an extra option that we can import “personas”, which are meshes with completely different bodies and faces (but that still fit the same rig), and will replace the base mesh and can be used with all the morphs to have even more variations.
I hope this solves your questions 
I will update soon with screen recordings of everything. I’m really interested in knowing what you guys think of this tool and finding friends to test it
Thanks for your interest!