Ciao Nick,
The Proloco beat me to the reply (thanks!!) , as I was doing some research myself... and can confirm from a few who I've called that they have told me that almost every car uses a different type of bike rack (the way they attach, as most models are hatchbacks here and so go on the back) and so they don't offer them, too complicated. I was thinking of trying calling a big bike rental place here in Florence to ask them, but with the various holidays these past 2 weeks, the one I was thinking about is taking a long weekend (and the 6th is another national holiday so most are not back to full operations until the 7th).
As also suggested above, a couple of friends of ours do just that: they have a Fiat Doblo' and don't bother with bike racks, just put the bikes inside. The car is like a tall box, you can put the back seats down to the front. If I remember correctly, they don't even take one of the wheels off and two bikes fit. But with luggage along, I'd suggest taking a wheel off. I think there is a similar model by Citroen, the Opel Combo and then, of course, minivans would likely work too.
You could also wait and see what car you get, then get a bike rack yourself once you know which model it is... but I think that would be more complicated!
Do you already know where you are flying into/out of? Have an idea of which area you'll stay in? Also, will you stay there for the entire 2 weeks as a base or planning to move around? While most farmhouses have the space to keep your bikes parked and safe, you might also consider looking into those that specifically target cyclists as guests through this portal:
http://www.albergabici.it/en/elenco?reg=toscana - We've stayed at
Villa il Poggiale which I remember has mountain bikes for guests, in case you also want to consider that possibility as an alternative.
If you're keen on doing trails in the woods, and stay off paved roads more often than not, then you'll be happy to know Tuscany has a high percentage of "white" roads (unpaved) as well as maintained trails in the woods by the CAI (Club Alpino Italiano) which are for hiking but often perfect for mountain bikes, too. How much % biking vs car day trips you think you'll do? Because you could also consider that if you stay close enough to a rail line, you can use the trains to get around with the bike to get to other areas to bike in for the day. Just trying to offer more possibilities - good luck!! ;-)