deb,
I'd suggest re-arranging the order of visits to Venice > Florence > Monterosso > Sorrento > Rome, as it's more efficient. Note that for the trip to Sorrento, you'll have to use the Circumvesuviana from Naples to Sorrento. It has a bit of a "reputation", so watch your bags closely.
Regarding your last question, my preference would be to stay in Monterosso; other opinions may differ. At that time of year, it should be fairly quiet in Monterosso and therefore a pleasant visit without crowds (the "official" start of the season in the C.T. is usually April 1st). Hopefully the weather will be good.