We have done this twice. You are only required to use points for the first adult in your cabin after that you may pay cash or use a combination (ie: cash for two and points for two- you can't split a person though and pay half points half cash for one individual)
Several people don't feel that using points for cruising is a good "value" for your points as oppossed to what you could get for them in Disney. I however love to cruise and for us it is a great value. We paid about 5,000.00 for our first cruise before buying into
DVC. Cruise number two was all points for DS myself and DH in a cat. 5. Cruises three and four are running me about $1600.00 for a cat. 6. (we use points for one adult and one child) so the savings for us is dramatic. We purposly bought the points to cruise as for us to spend 1600.00 a year vs. 5000.00 a year was a value. I am not interested in the hassle of renting out my points and since we have enough for us this works. To answer your question you would probably be best served with connecting cabins in like a cat. 10 to get the best value with that many people. The cash rates for the cruise are discounted but not as far as the earliest early booking savings, they become a "deal" about six months out as the cruise rates go up.