As Far as Necrons Go they should beef up pariahs or lower their points significantly in order to combat psychers as they cannot have psychic powers at least in the sense that the other races do as they are not warp sensitive, though there are a few loopholes in the fluff that might allow something similar to psychic powers. 1. It is Actually the C'tan who are opposed to the warp not the necrons though being servants to the C'tan means that they have to do whatever is necessary to protect their masters which would perish or at least be threatened by them if the were to cultivate warp sensitivity. okay i guess the first point does not really allow for psychic powers that involve the warp...so onto the second possiblity 2. In the Tyranid Codex perils of the warp are still in affect but in actuality represent their "brains frying" or some similar phenomena therefore the rules for the psychic powers do not necessarily have to deal with the warp they can deal with different phenomena from a fluff standpoint but could function the same from a rules standpoint...that being said, I would imagine that if the necrons were to have powers they would probally be orientated toward combating the powers of the warp, either reflecting or absorbing the powers of the other races, perhaps even their life energy in the case of the C'tan which i would definitely like to see the other two though if the dragon ever awakes the imperium will be in alot of trouble.. that is just in theory, In actuality items and abilities that represent their anti-warp/null tendencies
If anyone actually gets though this entire rant gets a cookie!