Your only real train choice is to leave Paris at 07:42 on the 31st, change in Milan, and arrive in Rome at 19:29. The direct night train that leaves around 18:00 is sold out.
You won't be able to take the direct Rome-Paris night train on the 3rd that would get you to Paris around 09:00 on the 4th because it is sold out.
Your budget airline choices are CDG-Fiumicino on Vueling, Orly-Ciampino on easyJet, and Beauvais-Ciampino on Ryanair. Check out the cost of a Paris-Rome roundtrip ticket on a national carrier.
You easily could take a day trip from Rome to Florence and back. The train ride each way takes about 1hr40min and there are lots of departure times all day long.
If you're going to spend any time in Rome, I doubt that you'll have time to go to Naples and on to Pompei. Depending upon how fast a train you take, it's 1 1/2 - 2 hours from Rome to Naples and another 40 minutes on the private Circumvesuviana train from Naples to Pompei.
Capri is out of the question.