I have dove with Divetech (under the previous owners), InDepth Watersports, and Cayman University Divers.
If small boat is really a priority (and I agree with others that it's more about the space on the boat and the number of divers in the group than the size of the boat), then InDepth and CUD would be great options. They usually go out of the West Bay Dock (as do many other smaller ops) and will go where they can to make things interesting. If they are going north, sometimes they will bring their boats around to Morgan's Harbour, which is also very handy to get at from the west side. They will pick up guests from the major condo complexes. I think all of them offer Nitrox these days.
Whether you get all experienced divers is not going to be something that you or the Dive Op can control, however. But I would say you are more likely to get less of them with the smaller ops.