If you pick up a car at Piazzale Roma, you must drive on the Ponte della Liberta' di reach the mainland: be warned that there are active speed cameras on that bridge which will deliver a fine if you speed over the 70 km/h limit + 5% allowance.Edited: 09 May 2018, 21:41
If you book through a car hire comparison site just be careful that you do not book with Goldcar !
Another one to avoid is Locauto/Enterprise.
Autoeurope is a broker offering discount prices and good service so you could book through them and go with one of the major rental companies.
Where will you be going though? Remember having a car at large cities is a liability. Plan your car rental strategically.