A baisc bolt on nitrous kit, if used within the specs of itself, won't hurt anything. What I mean is, you can get the basic NOS 5015 kit, advertised as a 75 shot and spray it all day with no problems. This is assuming everything is in good working order. Now, the problem lies with getting greedy. Sure you can get the 150 shot pills and swap them in real easy....with no other changes, you'll have problems. NOS did have a 150 kit out there, came with a bigger fuel pump...ie; adding the additional fuel.
That's not to say that the first time you spray it, the weak link won't show itself. For instance if you have a head gasket that is borderline now...it'll probably make itself well known soon. That's just a hypothetical example. If you have a 250,000 junkyard replacement bottom end, with some heads your neighbor gave you, a mystery swap meet cam, and a clutch that was sitting in the garage....all assembled by your uncles out of work buddy who use to own a shop......get the picture?
Big picture, and the safest answer...there are pro's out there that can take all the varialbes (these heads, that cam, xxx shot of No2) you have and make them play nice together. Most of us shudder at the thought of paying $xxx for a "tune", but those that have done it swear by it. If you're not willing to go the tune route, a wideband o2 sensor will tell you if you're in the safe zone with what you got.
-Good Luck!