So how would I go about replacing all the character models with a model of Mephiles the Dark ripped from Sonic 06?

I have a model of Mephiles (Xbox 360 one I think) and I have the blender add-on and p3d editor downloaded, along with the mod launcher and game of course) and I want to mod the game to have Mephiles replace every character (With him stretched to fit their skeletons of course so that I don't have to mess with any rigging or stuff), how feasible is this and what additional things do I need to pull this off? the wasp camera model also will get replaced but with Subsonic (The sonic looking thing in the Simpsons game) with his T-pose made to what the Wasp camera T-pose looks like. I got my models that I'll use from The Models Resource and I have some knowledge on what to do given the fact I am competent at figuring things out, I just need help here because I don't know where to start or how. I don't really like YouTube tutorials as I prefer reading what to do. In theory I can modify the game, in practice I have no clue what to do other then try and hope I don't accidentally manage to mess up very badly.