If I buy a 2 country Eurorail pass, will it include a night train (Venice to Nice), or does that need to be purchased separate? The reason I ask is there are many options for the night train, and if the eurorail did cover the night train which option would it be?
All a Eurail pass covers is a basic seat on a train....it doesn't include any reservations or supplements(which almost every train, except local regional trains seems to require these days), or sleeping accommodations, which can add significantly to the cost of your transportation.
As well, rail connections between Italy and the south of France are very poor. I'm not aware of any trains that'll give you decent sleeping accommodations, or make the overnight journey on that route without 4 or 5 transfers at godawful hours. A good place to check European train schedules is at the German national rail website.
Have used the Eurorail pass many times. Remember most European trains have six seat compartments with a closed sliding door for privacy. With a little bit of luck you can jump on the train and find an unoccupied car - either first class (Which Eurorail guarantees) or second class -close the door/pull the shades/put down the arm rests/ and grab some shuteye. Have never had to use a sleeper car which is VERY expensive and, in my opinion, not necessary.
I would caution agains "jumping onto any train." In Italy, Eurostar AV (ES AV), Eurostar Italia (ES), Eurostar City (EC), Intercity Plus (IC Plus), and Intercity (IC) trains all now require reservations, which can cost between 5 and 20 Euro extra, on top of your pass. And while the IC trains still have six-seat compartments, IC Plus and higher categories are all like aircraft now...3 or 4 abreast with a center aisle. As well, the final parts of your journey will be on "regional" trains, not exactly known for their comfort if trying to sleep. They're also second class only, so you're wasting a first-class rail pass on them.
I don't know where else you're travelling on this pass, but with a little research, you might find that purchasing point to point tickets, either when you're there, or in advance over the Internet, will turn out to be cheaper than a Eurail pass. Walk-up second-class tickets for the shortest daytime journey, Venice-Nice (about 8 1/2 hours) are about 62 Euro per person...first class, about 82 Euro.