If you are booking a cruise with points, at least one person per cabin must be paying the adult point cost - it can be a child, but I would guess this isn't a choice most would make.
As far as I know, you cannot reserve a category higher than 4 by using points - even if you are only paying points for one person.
We used points for our 3 children and payed cash for DH and myself for the member cruise. We wanted a cat. 4, and got it so I couldn't tell you if you could or couldn't go any higher.
I wanted a Cat 3 with all of my heart but couldnt get over having to pay more than double for it. I used points for one adult and toddler and paid for one adult and my 11 yr old. A Cat 4 was 3,000.00 after 316 points but a cat 3 was almost 8k I could take another 7 day in 2007 for that!!! Someday conceirge and 600 sq feet I can dream!!!