I personally wouldn't have given him my information. He has no rights if he was off duty and not in uniform. How do you know that he is going to use this information for waht he said? Here in Pennsylvania and lower NY we don't have to stop for an unmarked police car or un-uniformed police as we have had some situations where a man abducted a woman by saying he was an undercover cop.
I would call your city officials also.
I know of no jurisdiction in which full time peace officers leave their powers of arrest at the station house/barracks when they go home.
Just because they are off shift does not mean they cease to be police officers.
I'd also like to see a citation to statute that says you do not need to stop for an unmarked police car displaying police (blue, red, or blue/red) lights.
You absolutely must stop for a vehicle like that. However, what happens next is dictated by the appearance of the person getting out of that vehicle.
If the driver that pulled you over comes out in civilian clothes, do not not open any window and drive away while calling 911. If dispatch confirms the person is a police officer, request that a uniformed officer be dispatched as you pull over.
If the driver is in uniform, and you still uncomfortable, ask to see his credentials (official police photo ID).
Many states have completely unmarked patrol vehicles doing traffic enforcement. The excuse that the car is unmarked will not fly too well in court when the officer testifies that he had his blue lights on for you.