Hi tbtest, and welcome to Cruise-Chat as well!
Just to agree with Rick, if you book an IS gaurantee you will recieve just that as the guarantee with a possibility of an upgrade. Almost all Carnival interior cabins are twin/king convertible.
I would also echo Rick in saying first time cruisers should use the ship sponsored excursions to get a feel for the ports. I would exclude Grand Cayman if you really wanted to do it on your own as is pretty easy and safe. But it is a tender port so make sure you allow enough time and deal with a reputable tour company should you decide. They have some great independents there I hear for Stingray City. Or just take a taxi to 7 mile beach.
You certainly picked an awesome ship. Not exactly the itinerary I would suggest for first timers, but I would love it.
There are so many nice people on here with a wealth of info., experience and wisdom. Please hang around..with that I will say..