The day of your arrival, notify the hotel if you anticipate checking in after 6pm. You could risk losing your room otherwise. In a pinch, same-day reservations can be made in person at branches of the Office de Tourisme de Paris . FF is charged depending on the hotel category. Twin rooms are generally more expensive than rooms with a double bed.
Single occupancy is in many cases as expensive as double.
Breakfast is generally included in the room rate, but it's always wise to check.