Since I live near Fort Lauderdale I don't spend a lot of time at South Beach, although I work close to Haulover Park, commonly called North Beach. Kite flying is not prohibited on the beaches as far as I know. I would suggest using Google Maps satellite view to check out the beach near where you will be staying. South Beach has been built out so the beach is fairly wide, much wider most beaches to the north. That's the good. The bad is that South Beach is very popular so it can be crowded. Earlier in the day or before sunset are probably the least busy. Look on the satellite view for areas without cabanna's.
I can try to meet up with you, but be sure to connect with Rich (quaa714). He is in Normandy Island area of Miami Beach (between South and North Beaches) and likes to fly whenever he can. If you are going to bring just one kite I'd suggest a Mid-vent, but as Rich said set with standard and vented would be best. Lines 75 feet or so are goodt, but I find I often need to use shorter ones (40' or so) if the beach is busy. There is a Kite store with an adjacent grass area on the bay side of Haulover Park near the bridge over the inlet at the south end. It's a good place to fly especially if the wind is west to northwest. Hope to meet you.