I agree with slawecki–the travel alone to Venice can be a headache, esp. if you are coming from FCO. Maybe just stick to Firenze and Roma?

What if you kicked a couple nights in Roma, then traveled to and stayed in Firenze in the middle of your trip, then finished off in Roma?

I might even be inclined to stay in Roma for the entire week, there's so many things to see and you don't want to tire yourself out from the travel back and forth to Venice. You could check out Pompeii, I hear that's a manageable day trip....