how would i get the player biped to use a certain permutation? Example: You make the player a Marine, but you want the player to use the Sgt. Johnson permutations. how do you do that?

hs_doc is your friend
(object_set_permutation <object> <string> <string>)
sets the desired region (use "" for all regions) to the permutation with the given name, e.g. (object_set_permutation flood "right arm" ~damaged)