TECHNICALLY, all agencies that get Federal Funds (read - all of them) are supposed to use the ICS - and it was one of the failures pointed out by the 9/11 commisiion

NYC has a fairly complex set of rules RE ICS - the rules of which agency supplies the Incident Commander run about 1/2 page last I saw them - it can be summed up as Crime Scene = Police, Fire Scene = Fire, Medical = EMS, Some of the others become Highway Dept, or OEM, or DEP (water main breaks)

The REAL fun is when you have a fire scene that might be arson, or a crime scene that has a fire - Let me put it this way, there is a TAD of anomosity between the NYPD and FDNY (and the other way around) - There have been fisticuffs, Firemen arrested, etc