Good morning...first of all, I think you will have a great time renting a large home for the week. Second, we owned a 7 bedroom home in Emerald Island and rented it for several years before selling in 07, so we have a feel for both sides of the rental experience.
In all honesty, max occupancy BY LAW is 16 in a 7 bedroom. You can try to do the 17 but if a owner or management company finds out, the fine print on your contract allows immediate eviction with no refund. No different than trying to put too many people into a hotel room. You can probably get away with it, but there is a risk.
That said, there are a few 8 bedrooms around and you can find them on vrbo or through private companies like All Star. By owner us usually a better deal. 8 bedrooms should probably be locked up as soon as you are absolutely sure of the dates and folks going. It is very interesting that the homes that rent the quickest and have very high occupancy rates are the least expensive and the most expensive. One 9 bedroom in Formosa is booked almost solid and is 5-6K/week!
For that big a home and this huge a vacation I am assuming this is a very special trip for all of you and that the rental costs will be shared between families. If indeed this is a special trip and costs are shared, I would look for high end 8-9 bedroom home. Here a few I found quickly from both Management companies and owners. I have no financial interest in any of these. They are sure fun to look at cuz they are really nice!! I would try to get at least 6 baths so most of the adults have private ensuite baths...to me this is most important for a week of sanity together. Stay away from the 7 or 8 bedrooms with 4 baths...These are original 6 bedroom homes that have converted small area into bedrooms.
One other suggestion is to do side by side 4 bedroom homes...two pools, two kitchens...In October, slower season, You might find a good deal from a management company homes next to each other:
http://www.disney4villas.com/disney-villas-side-by-side/index.htm
http://www.orlando4villas.com/sidebyside.php
Have fun shopping!!! FYI the same home may pop up on different sites as owners may adverstise on many different sites as well as have a management company renting the home....Ted
http://www.discountvacationrentalsonline.com/orlando-private-homes/8-bedroom-fg8pw23/
http://www.discountvacationrentalsonline.com/orlando-private-homes/8-bedroom-fg8pw24/
http://www.vrbo.com/43286 (this is the 5-6K/week home...check out the availabiltity calendar!!)
http://www.vrbo.com/93876
http://www.vrbo.com/4733
http://www.allstarvacationhomes.com/res/estate-vacation-rentals.asp