jen,
I haven't read all the previous replies but in answer to your questions, here's my usual boilerplate on taking the Malpensa Express......
TRAVEL FROM MILAN / MXP TO MILANO CENTRALE
The quickest and easiest way to get from MXP Terminal 1 into Milan is to use the Malpensa Express (train). There are several departures each hour and the trip takes 52 minutes. You can buy tickets at the airport station either from a Kiosk or staffed ticket office, and the cost is currently €13 PP. Note that some departures have a change at Milano Nord Cadorna, so try to choose a train that goes directly to Centrale, unless Cadorna is your desired destination. There are a few important points to note. First, the M.E. has two routes, one to Milano Centrale and one to Milano N. Cadorna, so be sure to board the correct train! Next, it's necessary to validate (time & date stamp) your tickets prior to boarding the train, or you'll risk hefty fines which will be collected on the spot! You'll arrive at Milano Centrale on Track (Binario) 1 or 2, and will then have to walk into the main hall to either connect with another train or exit the station.
Regarding the Freccia trains from Milan to Venice, you do not have to validate your ticket for these, as you'll have compulsory seat reservations specific for that train. Be sure to note the Carrozza (coach no.) and Posti (seat no's.) which will be listed on your ticket.
The reason I asked if you'd be using a Vaporetto upon arrival in Venice, is that these tickets have to be validated also, but that's a bit different than with the trains. The ticket is electronic so all you have to do is hold the ticket against the machine. If you get a green light, you're good to go. You may find this useful - https://quickvenice.com/transportation/actv-tickets.htm .
From your post at 06:50.....
"Do you think i need to book Venice florence train now?"
As I mentioned in my earlier post, that's not advisable. The ticket is specific to train, date and departure time and if you miss the train you've booked, your ticket is worthless. While it will cost more to buy a Base Fare ticket on the day of travel, it's a safer option (IMO).