u can be any model in half life all u have to do is turn on cheats sv_cheats 1
then spawn the character u want to be, example npc_create npc_breen
then type cl_playermodel models/breen.mdl
u need the spawn the character first because it needs to precach the character. if u do this...
ya u can basically play co op thru out the whole game but at some pionts it just crashes cuz sp cant handle 2 players at some parts like at the end of the video where eli says feel free to walk around and then it crashes