Ok, let's go step-by-step here:
Paris-Italy: since you plan to visit Rome (which is rather far and not great (in terms of cheap tickets) rail connection from Paris), then we would probably advise you to fly Paris-Rome first and then make you way north. There are plenty of low-cost option for the route (EasyJet, RyanAir etc.), it almost definitely will be cheaper than rail (and much faster, which seems good for you since you'll have more time for Italy). If you don't want to fly, then the best option is just hunt for cheap discounted tickets in advance (either daytime TGV to Milan or EuroNight), you can check it out on
www.tgv-europe.com/enRome-Pisa: here (as well as on other domestic journeys within Italy) you have two major options: a) visit TrenItalia website and purchase in advance discounted MINI offer tickets for high-speed/InterCity trains. MINI offer can actually give discount up to 60% and if you know your exact dates, then it's a good and cheap option for fast journeys; b) you won't need to buy anything in advance, but instead use slower Regional trains, they are much cheaper (comparing to full fare ticket for high-speed/InterCity trains). For instance in case of Rome-Pisa regional train takes around extra hour/hour and a half comparing to faster high-speed trains, but cost only 18 EUR (which is less than half price of a full fare ticket for high-speed Frecciabianca train)
Pisa-Florence-Venice: same options as above
Venice-Switzerland: unfortunately it will be difficult to travel overnight here, while in terms of ticket you again seems to have two major options: a) purchase discounted direct ticket from TrenItalia website (discount offer is called SMART and unfortunately offered not for every connection); b) purchase domestic ticket from Venice to Milan (or even the border, like Como), tricks with domestic tickets (MINI vs. regional) described above, then from Milan (which is also a great place to visit) there are much more connections to Switzerland and more chance to buy cheap SMART offer ticket.
In terms of advice on itinerary around Switzerland...hmm... difficult choice since there are way to many things to see, but if to think "mountains", then probably something like Luzern-Interlaken-Lausanne-Geneva (from here it's easy to come back to Paris) or Luzern-Bern-Geneva, thus in both cases you tend not to travel far to the north, but stay closer to central and southern Switzerland(=most interesting mountains). For domestic Swiss travel best option is SuperSaver tickets (
http://www.sbb.ch/en/travelcards-and-tickets/tickets-for-switzerland/supersaver-tickets.html), Swiss railways tend to have such sales pretty often, so good chance you can catch one of those for July. regarding
Switzerland-Paris: same SBB website sells discounted international tickets (can take a look at
this)
For the question pass vs. point-to-point: kind of standard answer from us - pass will give you much more flexibility and you won't need to worry about missed connection etc., but at the same time smartly purchase point-to-point tickets (like MINI offer for domestic in Italy, SuperSaver for Switzerland etc.) will cost you much less in total, but might restrict you to particular timing and trains. So we would probably advise you just to visit both
TrenItalia and
SBB websites, check out tricks we described (maybe you'll have further questions etc.) and then make the decision about which option sounds better for you.