Can I expect a rental car to have a phone charger compatible with a US phone/phone charger?
The DC power outlet in most European rental cars is the same as that in US cars. You should be able to charge your cell phone or GPS unit as normal. The occasional traveler has reported that the outlet was a slightly different size, or that no outlet was present, so I suppose there are no absolute guarantees. You could check with the car company to put your mind at ease. You could also charge-up all of your electronic devices in the hotel room each evening.
I recently rented a car in Ireland and was able to use my car charger for my GPS and my phone with no problem. I don't think that Italy would be any different. The only problem would be if the car didn't have an outlet, but that is unlikely.
Don't worry about it. They're the same.