Venice during the film festival

I haven't been at that time either, but my understanding is the the actual festival takes place out on the Lido rather than in the city proper, so that might make a difference.

We were there just before the festival started two years ago. While we didn’t notice unusually large crowds in Venice, the obstacle we ran into was finding a water taxi after our flight. It’s usually easy to find an available water taxi at the dock, but because of the film festival, every taxi had been reserved. After almost an hour, our group of four was able to find one to share. So in your case, reserving a water taxi in advance would be a good idea if you won’t be using public transportation.

As pointed out, almost all of the film festival activities happen on the Lido, not in the historic center. You might not notice it at all.

Don't worry about it. Venice is the perfect place to start. I've been there during Carnivale and even if there are crowds at a peak time in the most popular places (Rialto Bridge, San Marco) it's easy to enjoy the rest of the city. Venice, like most of Italy, will be pretty crowded with tourists throughout September.

If you are really worried, though, consider flying into Rome and taking the train right to Florence, then Tuscan countryside/smaller towns, Venice, CT and Rome. Fly R/T Rome if you can't get a decently timed flight out of Venice. Most flights seem to leave in the morning, making it a difficult departure city. Florence to Rome is not a long train ride, so you could even backtrack a bit. I just think Rome is the least attractive option for the start of an Italian holiday because it's a big, bustling, crowded city with major sights - not to be enjoyed in a jetlagged haze.

While the movie stars might tend to take water taxis from and to the airport, there are also land taxis, land buses, and Alilaguna boats, so I don't think that's an issue.