The trip was last week and i'll share our experiences so if anyone runs into this problem you can try what we did...altlhough I dont suggest it!
We were able to take the EuroStar earlier than expected which would have relieved us of having to take a taxi - however the EuroStar was now running 1hour late and set to arrive at 11:45 into milano centrale. Once you exit the station there are signs for 'airport shuttles' and there are 2 or 3 shuttle companies heading to Maplensa. We ran and caught the 11:45 shuttle to Malpensa for 7EURO. this was not the Malpensa Express, but another company. We ended up sleeping in the airport and were up and ready to go for our early morning flight.
On the return is when we had problems. We landed at 10:30 and shuttles ran until 1:30am to Milano Centrale. There are two bus stops that both shuttle to Milano centrale and both are 7EURO. We arrived at 00:00 and had to sleep in the train station until 5:30 when we could take the first train back to Firenze. We saved a good bit of money but it was an odd experience. There were lots of homeless in the waiting room in the train station and when it closed at 2:00am the polizia kicked everyone out that did not have a valid ticket for first thing in the morning.
Great trip, fun experience but if you've got the money and the time get a hotel!! If you don't mind sleeping in the airport, Malpensa was not bad & full of people all catching 6:00am flights!