Hi Kevin,
This a great question- at first I assumed that fire extinguisher cabinets would be included in CSI division 21 "Fire Suppression." As it turns out- your contractor is right, and fire extinguishers and cabinets do fall under CSI division 10 "Specialties".
Thus, the cost of both the portable extinguishers and cabinets must be included in the MRc3-7 calculations.