If I have a rail pass and my times are flexible, do I really need a reservation from Rome to Naples or from Venice to Milan? Can I add a reservation onto my rail pass when I get to Rome/Venice? Not sure what to do.
First off, you most certainly can wait - there is no rush here.
You will need the reservation if you take a Eurostar Italia Alta Velocita (AV) train from Rome to Naples. These are the high-speed trains and it does travel at high-speed for this duration. These trains always require a reservation. The reservation will be about €18 per ticket.
The good news is that you can make the reservation even as late as just before you depart on the train. As you have a rail pass, you get no savings for booking the reservation in advance. There will almost always be plenty of seats left on the train, as these travel frequently on this route.
The reason why you need to pay a reservation fee at all, is more a suggestion that you are taking a higher-class train, and the reservation fee is really more of an additional cost to pay for the well air-conditioned and high-speed train - not a suggestion that the seats will all be full otherwise.
If you take a look at the Trenitalia schedules, you can see that you have the option to take trains other than the high-speed AV for this distance. There are some trains, such as a Regionale (R) train, that travel the same route and do not need a reservation fee. The downside is that the trains are not as nice, and they take much longer. The AV trains will get you station-to-station in 1:21, whereas the other slower trains vary from 2:10 to 2:30 for travel time.
If you wish to make the reservation at the train station, I would simply advise you show up a little in advance to either purchase one from a ticket window, or to pay at the automated machines. The automated machines let you purchase just a reservation instead of a whole ticket, but it is tricky to figure out. I had to have one of the station employees help me out on the machine, but he was very willing to do so and helped a great deal.
Venice/Milan is the same for reservations.
Thank you - that is what I thought, but it has been years since I traveled in Italy and I could not remember what to do. I will check Trenitalia just so I have an idea of the schedules.
For riding any of the train classes in Italy except the Regionale or IC trains, you will need to check in and pay a reservation fee. For the ICPlus trains, its 5Euro. For the Eurostar-class trains, its 15E-20E. From Rome to Naples, the high-speed is most likely the Eurostar-Alta Velocita. The fee is 20Euro. If you went P2P, the entire ticket, including the seat reservation, would be 39.90Euro. Thus, half your regular fare is an extra cost for having a Eurail pass.
For Rome to Venice, it could be either the ES or the ESAV. You don't need a railpass in Italy. P2P is always less expensive and a Eurail pass is always not as convenient as you expected on the premium trains.