So how do you change your player's head with a different characters head? I know that you type a command in the game console but I don't know what it is.
/model sarge/default
/headmodel merman/blue
e.t.c
I believe this question has been asked many times, so it's probably both in wiki and if you search the boards. Any character combination is valid, your opponents will only see it if they don't use forced models though.