Adding Skins - DS-GAMING-SE/Sonic GitHub Wiki
Getting Started
You can find the rig for Sonic here.
Import Settings
When importing your skin into Unity. Be sure to import it at 0.14 scale and with convert units off
Custom Super Form Model
Changing what a survivor/skin's super form looks like does require doing a bit of coding. If coding doesn't scare you away, you can find more information here