First re Dorchester, I believe you can bring your own dogs, although they can't stay in the cabins as those are owned by individuals letting the Operation rent them. I used both a light 12 and a 20 last year and no one said a word, both were SxS and I do believe they limit use to O/U or SxS. Some of this is safety related. Put yourself in a guides shoes, seeing new guests every coupe days and not knowing how safe they are handing a gun.
Birds are released all through the season , not planted. We actually found a nest and quail eggs inn the field. They do use a jeep to get you out to the field but we walked the rest of the time.
The 12 vs 20 and smaller debate. About 15 years ago I was invited to hunt customers private plantation. The first year I brought a little J Dickson boxlock 12 and 7/8 oz loads. I was gently reprimanded. I tried to explain the light load etc and it largely fell on deaf ears. I think the perception is if you are using a 12, you are concerned about taking a lot of birds, whereas if you are using a small bore you understand it is NOT about the numbers of birds shot but rather the experience. The next time I was invited I brought a 20 O/U and in the end left it with the 2nd in command. Funny part of the story is, we hunted on horseback and as the only Yankee that had been allowed on the plantation, I was given the biggest damn horse you ever saw to ride. It was a chore getting on and off him all day.