We just got back from Italy. The line to go through Passport Control at Malpensa was extremely long, and we were in the front of the plane. Took us a full hour, if we were further back in the plane would have taken even longer. We landed early in the morning, and I almost felt that perhaps no one was at Passport Control that early to work. So, even if you're not checking bags, it would be hard to know what train you'd be able to make.
We took the train back. Yes, all the tracks in Milan Central are on the same level, so the transfer was easy. However, I personally would allow for more than 15 minutes, because according to what I saw on the boards, there were delays on some trains. 15 minutes is enough time to walk from one track to the other, but if the train is late, 15 minutes won't be enough time. If you miss your train from Milan Central to Florence, you might have issues getting another train. The reverse direction is not an issue - if you miss your train from Milan Central to Malpensa, the ticket is good for 4 hours for any train.
A few years ago, we were going from the airport to Genoa. Since we are never sure what time our plane will actually arrive and the time needed to go through passport control and getting luggage, we didn't pre-purchase our tickets. Turned out when we got to Milan Centrale, all the seats on the next 2 trains to Genoa were sold out. So, not buying tickets in advance can be an issue. Probably should have bought tickets Centrale to our final destination once we knew what train we were making from Malpensa.