I would assume that most SOPs are considered of a 'sensitive' nature (meaning not for public release aka here). I would think if you would officially ask departments from larger cities they would be willing to provide guidance to assist you better.
Some key words- extraction team, casualty collection point, hot zone, warm zone, triage area, treatment area, transport sector.
Any way you look at it a mass casualty incident, active shooter, violent attack will be a cluster fornication and will tax even the largest department. Being able to work closely, communicate directly with other departments around you (fire and EMS if not together) and LE will be critical.
Most of these runs require fire/EMS to stage and wait for our copological detectors (love you guys!) to 'clear' the scene for us, per SOP's, so don't be afraid to hit the supply closet and stock up the truck before leaving the station. Some life saving skills can be used inside the warm zone but obviously it would be better to not be in that area, you are not going to do CPR but open an airway, apply a tourniquet or something else quick and easy.
Almost forgot; yes we train and coordinate with our city LE, county LE and several smaller city LE departments.