Sorry that I have no answers for you, I am doing a weekend trip the week after you. I would be interested in your trip report when you get back. I will be staying at the st. regis sat/sun and the hilton on mon. so anything you can pass on to me about the shuttle/cab situation would be great.
Have a fantastic time! and wear comfortable shoes since Rome is a great walking city--the last time I was there I several miles a day.