trip to northern Italy

If you're flying into Milan, you'd probably do best to take the train(s) down to the Cinque Terre. See Trenitalia.com for schedules.
As you leave the C/T, you'd take the local train to La Spezia and transfer over to the regional train over to Florence.
After you do the tourist thing in Florence, I'd suggest you rent a car and head south into Tuscany--staying at an agriturismo. Spend your days visiting great hill towns like Volterra, San Gimignano and Certaldo.
Turn your rental car at Florence, and take the train to Venice.
Then take your final train trip back to Milan.
Getting a driver to take you around Tuscany would just be too expensive. The region is easy to travel in, and the roads are well paved. They are just a little crooked, however.

I think Florence, Venice and CT are easy enough to understand but what is it you want to do in Tuscany beside seeing Florence?

It is hard to recommend if you should have a 4th base South, North, East or West of Florence or just use Florence to see other areas in Tuscany without knowing your interests and what you desire to see/do.

As a rough thought:

CT 3-4 nights, Florence 3-4 nights excluding any day trips, Venice 3-4 nights excluding any day trips, figure you will end the trip with 2 nights in Milan since you are flying out of there. That obviously leaves you plenty more time to do and see other things, not only day trips at each location but other bases if you want.
Tuscan hilltowns, Italian Lakes, Dolomites ; most anything in the northern half of the country could be added into your timeframe.

A car would be as useful in Florence, Venice and CT as it would have been for you when you were in Rome ; in other words not at all.
For seeing certain areas in Tuscany though it definitely makes sense and can be easily done.
Not sure why you would need or want a private tour with a guide but that is your call, I do know the name and email of one who I used for a ride while in Tuscany.

Is the trip for this year 2017?

You could take the train from Malpensa to Verona (change in Milan) for a night or two, then on to Venice. Bologna is a good base for day trips by train to Padua, Modena, Ferrara, Ravenna, and others. Then Florence to "wallow" in Renaissance painting, sculpture and architecture. I'd spend a couple nights in Siena, maybe use it as a base for other Tuscan towns. Or stay in an agriturismo with a car and visit that way. With a car, add some time in Umbria, as lovely as Tuscany and less touted. Include overnight visits to Orvieto and Assisi. I'm less than a fan of the CT these days. Some trails seem to close every year and there are reports of hordes of tourists, mainly day-trippers from the huge cruise ships. Instead of the coast, consider spending a little time at one of the lakes. Lastly, a night or 2 in Milan before flying home.

Note: If opera season is still on, skip Verona - chances are you won't be able to find a room and it will be crowded. Or you could visit near the end of your trip instead.

This would be the path I'd take.

Milan>Venice. Train.

Venice>Tuscany. Rent car as you're leaving Venice. Head as far south as you wish to visit, creating a loop itinerary.

Tuscany>Cinque Terre. Visiting Pisa or Lucca on the way. Drop car in la Spezia. Train to village of choice.

La Spezia>Florence. Train.

Taxi to Florence airport.