As it stands now, Annapolis dispatches its own police department, so going with FiRST for police services would not be a big deal. And new radios would be 700/800 P2 compatible in any event, so switching zones would be no more complicated than it is now. The other thing is that the MSP and the Capital Police are on FiRST, as well as DNR - there actually is an Annapolis police boat as well as harbormaster boats. In the end, it probably will depend upon who offers the best terms.
But for sure, the fire department would stay on the county system. In fact, I am surprised that fire protection has not reverted to the county, given how expensive it is to duplicate administrative positions. Annapolis is not like Baltimore City - a jurisdiction as large as a county.
As far as the county as a whole going onto FiRST, AACo is too large thus would require spectrum that simply is not available. But, ISSI between the two systems - pretty sure that will happen.