1) Yes, you should get reservations on the Paris to Florence night train. Especially if its the 227 Palatino that leaves Paris-Bercy at 18:59. This run has no train changes and can be quite popular. Book as early as you can. Note also that the night trains into Florence do not terminate at the main Florence SMN train station. They don't even stop there. Instead, they terminate at Florence-Campo di Marte. Campo di Marte is a commuter station in the morning. You should be able to hop on a quick commuter train to Florence SMN or you can take a taxi.
For Rome to Milan, you have choices. The regular non-night train run takes 4.5hrs. There is a night train departing Rome for Milan at 23:00 with no train changes but the run takes 8.25hrs or double what the regular trains take. Same tracks. The train schedule shows the night train sitting in Orte for 5.0hrs before proceeding again.
2) I would suggest that you can easily get the Paris-Florence train tickets in Paris but the sleepers might be sold out by then. You won't be able to buy tickets for Switzerland or Italy in France.
3) For tickets in Switzerland, you can buy these in advance on the Swiss train website. www.sbb.ch/en/index.htm You can easily get the Bernina Express and Glacier Express tickets once you reach Switzerland. If you have a Swiss pass, as opposed to a Eurail pass for Switzerland, you may have to pay a seat reservation fee on the Glacier Express. We had to with ours. Not sure about the Bernina Express.
