You can support our forum in a small way if you use rentalcars.com to make your reservation through our website here: http://www.discovertuscany.com/tuscany-car-rental/car-hire-in-tuscany.html since we'll get a small commission from them. In addition, I've personally used it for renting outside of Tuscany and the rental was through a major rental company, all went well with the reservation and had no problems at all at time of pick up. I'll definitely go through them next time we need another car rental.
I've never personally used the other company you mention so cannot vouch either way, sorry!
The Car Rentals Terminal is only 500 meters from the terminal building and is directly connected to the Airport by a shuttle bus service. The shuttle bus stop is located just outside the Airport Arrivals area. You can also reach the Car rentals Terminal on foot after 5 minutes walking only.