Yes, that is absolutely fine. Rosemary is also used for protection. The brew can be used for both protection and clearing negative energy. Humans contain energy, so does anything else, if it can cleanse human energy it can cleanse most any things energy.
Yes, that it one of the many uses for rosemary. Remember, if it feels right to you, chances are you should try it out. Might want to put a pinch of sea salt into the brew as well (sea salt is also an excellent cleansing agent).