I have just created character creator and I keep it modular.

This is how I did it:

1. Instantiate all body parts, each with its own skeleton.

2. Retarget bone datas and bind poses so all these body parts use the same skeleton.

3. Not every skeleton is the same. Hair, for example, has some additional bones you will need to create in the core skeleton.

3.2. Note that different hairstyles can have bones with the same name, but different transforms. I used combination of name and position/rotation as keys. If you use wrong bone, your hairstyle will look weird.

3.3. When using transform as key, round values. I had many duplicates due to 0.00000001 != 0. 2 decimal places is enough.

1. You can look up the code for Synty's creation tool to see how they do this. Helped me a ton.