It's time for a bit of math. A Eurail pass for Italy that can be used 5 days out of 2 months will cost you $249. Now add 18Euro for each Italian Eurostar run which is what they do in Italy. You have 3 (Venice/Florence, Florence/Naples and Naples/Rome). You now have invested $303. Next, the Circumvesuviana train that runs from Naples to Pompei and Sorrento will not accept your Eurail pass. It's a really cheap ride anyway. For your Florence to Pisa/Lucca/Sienna runs, these are all very inexpensive runs on the Regionale (R) train. No extra hits on your pass but they are so cheap, P2P makes the most sense.
Now, the P2P costs for everything but the Naples/Pompei/Sorrento runs. The costs are for 2nd class tickets on Eurostars where available. Venice/Florence ($40.67). Florence to Pisa ($7.05). Pisa/Lucca ($3.02). Lucca/Sienna ($11.96). Siena/Florence ($7.68). Florence/Naples ($73.03). Naples/Rome ($40.67). Your total point to point with 3 Eurostar runs is $184.08. Whew!!
You can lower your P2P costs even further by traveling on the IC or ICPlus trains on the runs used by the Eurostars. But, they don't run as often as the Eurostars. For example, Florence/Naples would be $49.10 and not $73.03. Travel time is almost the same. From Naples to Rome, it would be $27.07 instead of $40.67. Also, your additional pass fee would drop to 4 Euro from 18 Euro that you would pay on the Eurostar.
You can easily buy the P2P tickets for Italy in any station. It's best to use the automated ticket machines. These machines will prompt you in English and will accept your American credit card. The machine will assign your seats as well. Just be sure that you know the dates an times for your travel before stepping up to the machine.
You don't need a railpass in Italy.
