First, I recommend Key Largo/Tavernier. Vero/West Palm/Ft. Lauderdale are deeper dives, in stronger currents and better once you have more experience. The keys reefs are beautiful and all of your dives can be 40' or less. By far the best choice for new divers. For the first day, I recommend an op that will put a dive master in the water, at least for your first day. If conditions are benign (clear and not much current) then no reason you cannot dive on your own the second day.
In the keys, 6 tanks in 1.5 days is not a problem if you are basically fit. You will sleep well!
I use nitrox on all my dives deeper than 20'. I am 55 and it does seem to lessen fatigue. Also, worth it to reinforce your training.
Lots of great ops in the Keys. I would go with a smaller one as you will get more personalized service and assistance with gear, etc. Rainbow Reef has a top reputation. Another is Quiesence, which only runs boats limited to 6 divers. At Quiesence, however, you will need to call them to arrange a dive guide in the water with you. However, if you have already contacted a shop and are happy, stick with them by all means.
Most Keys ops do not put a divemaster in, or if they do, it is for the whole group. Check with the shop. For your first morning dives, you might consider arranging a personal dive guide who is dedicated only to the two of you. That can give a good comfort level and also is nice because you have someone familiar with the sites and who can help with navigating and bouyancy if necessary. After that, it is up to you whether you want to dive on your own. Check with whatever shop you use, as they all can arrange this.