When you check your cats in at the airline counter ask if they can load them facing each other- most airlines try their best to accomadate these type of requests and it may be nice for your cats to see each other- I know that I have received this request before, and I have gone out to personally make sure it was done.
Sadly, travel agents no longer have much influence on the airline industry- your best bet is always to call and confirm reservations yourself. I can not tell you how many passengers are told that they have pre-reserved seats only to find out that the travel agent didn't enter it correctly into the system- the ticketing agent then has to explain why you are in a middle seat 20 rows away from your companion on an oversold flight- not fun for anyone!
Your hotel in Miami should be easy to check on. Where are you staying in SAP? I really like the Microtel near the airport, but it's pretty far from town. In SAP the Gran Sula is nice too.
The Yahoo group is going to great for you as it is filled with lots of ex-pats that live on the islands or go back and forth. It is also a great way to become aquainted with the people who will be your neighbors!
Make sure when you pack up the cats you put a bags of their food and a bottle of water on the top of their crates (in a ziplock, taped is usually just fine). Also make a name tag for each of the kennels with the cats name. Whenever possible we talk to the animals and using their name can help keep them calm. My office is overlooking the ramp and I often have pets wait with me to keep them from the heat/cold and noise of the ramp. Heartworm is something cats get, but it is not common where I am for cats- the same medicene I give my dog is also acceptable for cats. You will want to set up a mail account at Jackson or Hyde and you can get most meds online this way. Good luck and keep us updated on how it goes!