I've done groups of 10 all the way up to 25. I always book a buffet since there are so many palettes to please. I would aim for first sittings in the meal times, so say 7:30 or 8 am for breakfast.
When I booked the 25 people, Grand Gatherings was still open and while we were told that our party may be split up, because they gave me the option of the first or last sittings and we took first, we were all able to sit together.
The places I would focus on if you want to sit together is Cape May Cafe or Ohana's.
At 1900 Park Faire we were divided up into 3 round tables. Next to each other but just not the same.
Over all Ohana's was the best experience and the only place we were able to get a pic of the whole group with Pluto.