Well Curris, I a while back, I asked the question "what other things can you do with a spirit besides just listed powers; that is, what would a spirit be able to do by simply BEING a spirit in a particular domain." It was supposed to get people thinking about other ways to use spirits by thinking outside of the box. In the example I gave, my players summoned a Force 4 Wind Spirit and had it vaporize a army helmet full of DSMO laced Gamma-Scopolamine, by sucking the liquid into its windy form. Then the spirit glided through an army camp. As it did so, the soldiers were breathing in teh GS, and dropping like flies.
Now, as far as I know, none of that really jibes with listed powers, but I could certainly see it being feasable for a critter that is essentially air on the material plane. So in the case of the Halon, I could certainly see a wind spirit being able to push the gas back. However, if the entire area were flooded with halon and it had had a chance to work on the oxygen in the room, then I think you would HAVE to use it as a Guard power since very little oxygen would have survived the gas. As I've been told (and I'm sure ROOTless will be able to clairify) halon actually destroys oxygen, or at least renders it totally unusable. Therefore, if there had been halon flooding the room, there would be no breathable air left and the spirit would have had to "make" it by using a power.
Of course, I still like playing spirits a bit more losely than canon rules seem to dictate. To me a spirit isn't really a critter, but the reflection of a place/event/ideal/etc. So there are bound to be things that they can do that aren't covered in their power list. For instance, have you ever thought of asking a Hearth Spirit for juicy gossip?? After all, it is the manifestation of everything that happens in the building, so therefore would know ALL about what goes on inside the building. You wouldn't even need to make Int. Tests if you asked it directions. It KNOWS the building because it IS the building.
Think about it and have some fun with the idea.
Gabriel