Most people who are considering the Amalfi coast area (including Pompeii) prefer to stay in Sorrento. To answer your question about Pompeii, the way it gets played up you'd think you're about to walk into an abandoned town pretty much like it was 2,000 years ago. Having visited I was disappointed; a lot of it is little more than brick walls here and there. I thought the ruins at Herculaneum up the coast were much better preserved.
For overall advice, I'd go with the consensus so far and save it for another trip. There's more to see in the region including the spectacular Amalfi coast, Capri, etc. Wait until you have 3-5 days to spend in the region. As for overnite trains, the places you are talking about are too close together to take an overnite train (except going from Venice onto Paris). Since most of your itinerary is in the north of Italy (relatively speaking), I'd suggest visiting the hilltown region of Umbria and Tuscany. IMHO less is really more when travelling, as far as moves go. Every time you move you pack up, carry all your stuff to the train station, ride to your destination, then have to find the new hotel and get squared away. Even a 2-hour train trip is going to take you until noon or later when all is said and done. So what I recommend is extnding the time in Rome to 3 days, the time in Florence to 3-4 days and using the extra time to explore some of the region by train/bus/tour-group. You could go to the wine region on a tour, visit Sienna, etc.