Most limo services charge an extra fee for night pickups. Usually after 9 pm until 7 am is considered night. I've used www.limoservicerome.com and they have a 20% surcharge for nightime pickups.

bisabella-I frequently use these airport hotel shuttles at different European airports when I'm traveling on my own, and I've never waited more than 15 minutes for a shuttle-usually more like 5-10 min. Because they aggregate people in a minivan who are going to hotels in the same area-it shouldn't be long at all to get to your hotel, even with a minivan of 6 or so other people-and it may be that your hotel is the first drop-off. They're very easy to use-just go the desk, pay your money, and you're given a voucher-you'll then wait until the shuttle going to your particular area is called. As you can see, it's convenient and most inexpensive, if you don't want to go the train route, then have to transfer to a taxi.