By what measure are you determining that "it's only ok to leverage cops against nazis, rapists and stalkers?" I don't necessarily condemn a person for doing so, as anarchists' own track records for dealing with rapists and stalkers leave something to be desired, and nazis would happily wield the force of the state against anarchists, but I do think this brings up some troubling, or at least problematic questions.
If we are speaking of practicality over ideals, I think that is fine in some circumstances, but I also think there should be more to making a choice like that than that it is okay. As I wrote above, I have known people who didn't feel their scene/friends/movement would have their back in dealing with a stalker or with a rapist, and as much as I deplore that that is the case, I have seen shit get handled badly (and certainly have fallen short myself sometimes) over the years. I have a hard time faulting a person socialized by our civilization and seeing the reality on the ground going to the police in such cases.
In the case of nazis, I think it gets even murkier. Nazis would use state power exactly because they covet state power. Anarchists want to destroy state power. While I accept the 'by any means necessary' perspective on fighting fascism, using police (and the courts) does create an interesting paradox in the case of attacking groups who would assume those roles. Other places I see this tension arising would include persuing the prosecution of cops (yes, it puts the individuals in a hardship, but ultimately it reinforces the legitimacy of state power). Of course I am all for people using legal channels, if that is their passion, to stymie the mechinations of state and capital, the legal challenges used in many campaigns come to mind, but I think that if we are to use those means in our fights, we need to be very clear that they are tools and we are not recognizing them as legitimate, simply available and convenient.