+1 for Quiescence. There are a lot of good operators in the Keys but these are at the very top. Nice boats with small groups (6 max), great captains, full assistance with gear (which few or no other operators can match), and very safety conscious. They are a few dollors more than other ops due to the small group size, but worth every penny. They are also very good at putting together groups of people with similar abilities and then offering a flexible selection of sites. When you call tell them what you are interested in.
They do not offer a standard guide in the water with the group (no Keys operator does, that I know of), but can arrange a guide for you if you give enough notice. I don't know what the fee is.
You might consider going without a guide. 20 dives should be enough for you to be comfortable on the Pennekamp reefs. If you select one of the standard Pennekamp reef dives (that would be the best for your experience level), the conditions are shallow (25-40 feet), vis is usually good (40 to 100+ depending on conditions), and they are usually low-stress situations. If you want, you can stay near the boat and enjoy great reef diving. If you want to roam a little, the usual procedure is to begin by swimming up-current (there is usually a slight to moderate current), making your return to the boat pretty easy. These are very good conditions for both enjoying yourselves and practicing basic skills.