I agree with Dave. Call Thrifty and see what they can do for you. I suspect that you will end up waiting for a "full bus" before they leave for the rental agency. There is also the time between buses to consider. We waited about 30 minutes for a shuttle to show up. Maybe a taxi to the rental agency is your best bet. Yes, its going to cost you $25 or so, but you are not going to make it if you wait for the shuttle unless you get really lucky on a shuttle showing up as soon as you need it.
Make sure the cruise line knows you have an early flight and be ready to get off as soon as they announce self debarkation and you stand a good chance at making it using a taxi to the rental car place.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007