Try this website for hotels. I found it had great info and visuals.
It is worth going over to Pisa for an afternoon. You've got see the Leaning Tower. Fly into Rome and out of Venice so you do not have to fret about getting back to Rome only to fly out. You picked three cities that are tourist friendly. Each one is completely different and full of things to do. Have a gelato every day!
We visited Venice last year September and loved it. Although for more than 3 days may get a bit boring.
We stayed at Al Campaniel
It was a gorgeous B&B in a fantastic and less touristy part of Venice - which means the restaurants weren't so ridiculously priced as everywhere else in Venice. Getting around is really easy too.. by foot and boat.
Can't comment on the rest of Italy as we also planning on going in May... Have a good trip.
If you only have a week, I would skip Venice and stick with Rome and Florence. Rome can easily take 3-4 days, and Florence can easily take 3. And you don't want to rush when you're in Italy
You'll want to stop in Pisa in between Rome and Florence.
However, if you're looking for some serious shopping, then a day in Milan is a must.