Yes the rules for npc's are different and no they are not written down anywhere. Npc's don't need to be approved and don't count toward character,df. or iq limits. When fighting they can be killed and you can call hits on them, however it really depends the user of the npc and what level of npc it is. Like an one hit kill no one will care, but with a more developed npc it will be expected that you follow the standard character rules such as no hit calling.