The restaurants are either next to each other or one floor above. We were turned away at Normandie at 6:30ish as it was packed. Told to return in 30-45 minutes. We enjoyed the food and service there. If you get to any of them by 6, you should be fine. Normandie was the "hot" one on our cruise.
There were no reservations for the MDR on the Beyond. However, if you are in a large party, let the staff know when you get on board.