This, coming to it without knowing anything about modding GTA V is a hard read, but after some YouTube videos, trial and errors, a few days... I made a working Trevor Replacer!
I am absolutely thrilled, thank you so very much!
EDIT: I made a working PED for Trevor, 3 actually, but the same peds have stiff thighs on Franklin, they don't move like the rest of the model instead just stretching out when the calf moves. As for Michael thats just a whole mess, only shows a bald head and hands, they are the correct ones at least. I find it odd that each player behaves differently, they all work on Trevor no problem it's weird.
EDIT:/Update: For Franklin, I tried swapping skeletons with another peds, same problem. I've tried replacing the lowr files but found out for the peds I've tried so far that's for the hands. Have now found out the problem[?] is in the uppr files, somewhere.
EDIT:/Update: Some custom peds are made differently and will cause issues, others work fine, in game peds all appear to work perfectly.