Nate,
Rail Passes are no longer the "deal" they used to be. In most cases, it's far more cost effective to pre-purchase tickets online, although doing so will "lock" you into a specific departure time.
Keep in mind that Rail Passes DO NOT include the reservation charges which are compulsory for the high speed trains in both Italy and France. Especially in Italy, if you're caught without a valid reservation for the train you're riding on (Rail Pass or not) you'll likely be fined on the spot, with fines starting at about €60 PP! Pass holder reservations are limited in France, so if they're sold out for the day you plan to travel, you'd have to buy tickets at full price.
The method I'd choose in the same circumstances......
- Florence to Milan: Travel via the new Italo high speed service from Firenze SMN directly to Milano Porta Garibaldi (departures about hourly). Trip will be about 2 hours and they currently have tickets listed at €20 PP. You can buy your tickets online.
- Milan to Paris: Travel via TGV high speed (~300 kmH) from Milano P.G. to Paris Gare de Lyon. There are numerous departures each day, but only a few DIRECT trains. Your best bet is probably a departure at 08:50, arriving Paris at 16:11. You'll have to check the SNCF website for prices.
Happy travels!