Regarding your questions (based on 8, one week long trips diving with SC)
1)Tolerance to cold is a highly personal thing, so YMMV. That said, I usually go to Nassau during August, but on the one trip I did during May, I wore: a 3mm full suit, with a 3mm vest, and a "beanie", and was comfortable. However, on the same boat, there were people wearing 5mmsuits, with vests and hoods, and one guy wearing a 3mm shortie. Also, if you plan on doing 4 dives a day, I'd suggest thicker protection.
2) The boat anchors, and you return to it. The first dive of a "2 tank" is guided, but for the second dive, it varies with the DM.You can do your own thing, or follow the DM, but in some cases the DM will be in the water for a while, then he/she exits, and the divers are on their own/free to do as they please.
3) I think that they require buddy teams, but I'm not sure how tightly they enforce that once in the water.
Additional thoughts: SC's is a very large, very well run (IMO) operation. They do run on a relatively tight schedule, and need to be back at the shop according to that schedule. If you happen to be on a boat with alot of people from cruise ships,things can be a bit rushed, as they have to get back to the ship by a certain time.
That said, I did, and hopefully will continue to enjoy diing with SC.
Enjoy your trip!