Here is my opinion for a travel schedule. When you arrive in Milan catch a train and go to Venice that morning. Spend three nights in Venice (the nights of the 2nd 3rd & 4th) After breakfast at Mid morning on the 4th take a train to Florence arrive around noon. Spend 2 nights in Florence (5th & 6th). Catch a train back to Milan on Friday afternoon the 7th. One night in Milan.
Recap: a half day in Venice (2nd) + two full days (3rd & 4th), Half a day travel, half day in Florence (5th) + plus another day and a half in Florence (6th and 7th) then travel to Milan giving you a late afternoon and evening in Milan.
10 AM flight on the 8th. Five people sharing a cab or shuttle would be easier (and cheaper?) than the train or bus from Milano Centrale.
Reasonable priced train tickets are available in advance from Trenitalia. Or course buying an advance fixed time train ticket for the morning of your arrival flight could be a problem if your flight is late. Other segments that would not be a problem. Go the the web site roninrome.com for great info on how to buy train tickets on line.
Get one of the Rick Steves Italy books for good advice on hotels, restaurants & nightlife.
Okay I just re-read you posting and realized my suggestions may be to more touristy destinations than you are wanting. But in early March they probably will not be very busy with tourists. But even if you go to different cities in Northern Italy this schedule would still be a reasonable since you only have a week.
However a week in Italy is still way better than a week in most other places!