The only thing I can tell you with absolute certainty is that some models may not have the "SP" tag in their .upl files. Without this Single Player (SP) tag, those models will not show up in the single player game.
Adding Menu="SP" somewhere in the .upl file of the suspected skin, separated by commas, will likely add that character to your roster during the game. I don't believe the game will select any non-OEM characters as opposing teams or team members, but you can have them on your team.