One of the main reasons they limit the number if people with the person using the GAC is space. If there is a different waiting spot or boarding area, they can't accommodate more than 6 because they physically won't fit.
For rides, there are very few ride cars where that many people will even be in the same ride car together. Many ride cars hold only 4-6 people, so they would have to separate into different ride cars anyway. Because of those things, they can't be required to provide an accommodation that is not physically possible.
I agree with ireland_nicole - with that many people, you can create a pretty good buffer zone. And a touring plan like TourGuide Mike would allow the whole group to stay together because they would be just following the usual waiting and boarding areas. It would be much more predictable - if 15 people are using Fastpasses or the usual boarding area, they won't need to be concerned about what accommodations are available.
Many people have also found their wait using a touring plan was 10-15 minutes for most attractions. The touring plans help you to be in the quietest part of the quietest park. That helps with attractions and things like just getting around, which can be even more if an issue than attractions.
They would still need to aware of how they are going to split up for ride cars. You can ask the CM at the entrance how many people per ride car will fit so that you are not trying to do seat assignments at the boarding area. The easiest would probably be to have a sort of set group of 4 people who will sit with him ( they may have to divide into 2 rows).
And, I would strongly recommend against trying to keep a group of greater than 8 together for touring. We have toured with groups of up to 14 ( not using a GAC for more than 6). Once you get to 8 people, it gets really hard to keep them all together. We only did it for attractions where we had gotten Fastpasses and would all be in line together. My DH was the line leader and was very familiar with the park so he could navigate for the group. I was the end of the line people wrangler. That was a difficult job because I had to make sure everyone stayed with the group. That was quite difficult because as people saw things along the way, they stopped ("I nee to stop just a minute to look at this tee shirt...I've been looking for this...I need a drink of water...there's a bathroom. I'll just be a minute."
It was not easy getting that many people on when to eat much less where to eat. There was a commercial a few years ago about herding cats. Touring with that many people was very much like herding cats. The days we broke into smaller groups were much more enjoyable and it was fun to meet up with other members of our party later and hear what they had been up to. In our case, we had a core group of 2 people without person with a disability and switched out the other members of the group to make no more than 6 to allow everyone a chance to tour with a variety of people. That may not work for some groups because their person with a disability needs a very consistent small group.
