I've seen some loose fitting rotors over the years so that's not unusual. How tight were those lug nuts on when you first removed the wheel? Make sure you torque the lug nuts to 150 lbs/ft (that's what's required for the s550, I assume its the same for the s650). That should sandwich the rotor...
I always ask for them to email the window sticker. If it's in the dealers inventory (which, if its for sale, it will be) you can also pull it up on the Ford Inventory website. Sales people will say anything to get you into the dealership where they believe they'll be able to control the...