As others have pointed out, pickings may be slim because of the World Championships being held in Tuscany. Friends of mine have rented carbon road bikes from FlorenceByBike.it and were pleased with the bikes, the staff and the prices.

The comment about restricted driving zones is spot on. You need to have a permit to drive in them: if caught w/o one, you're subjected to horrendous fines. Florence can be very confusing to navigate in.