I've done it quite a few times with them - this year I have just 4 nights booked in NY with them, we have booked the Orlando part independently. I don't think there are a minimum number of nights - they have certainly never mentioned it and this is the third time I've done independant stays. If you are booking a car you could just book the trip as a fly-drive - you're not expected to make any room bookings with them then.