Are you asking if you can book the 13:26 out of Mestre rather than wait for the 13:44?
I think the 3 minutes of the first connection is way too tight to count on. If you book the ticket all the way through Trenitalia it puts you on the 13:44 to Treviso and I think that's the right choice. If you are booking it separately for some reason the 13:44 is the logical choice. It allows your train to be a couple of minutes late, and the station to be busy and you can still wander to your track a couple of minutes before it arrives at 13:41. If you book the whole ticket through Trenitalia as one ticket they are responsible for getting you to your destination so if the train into Mestre was so late you missed your connection they are required get you the next ticket to Treviso to finish your journey.
Get the Trenitalia app and you can track the Treviso train and probably get its track number before exiting your train at Mestre.
Your instincts are right about the food on the trains. Even on fast trains the cafe car should be a last resort. I would pick something up in Florence and take it on the train with you - buy a beverage in the cafe car if you don't have anything. On the fast trains you get a tiny complimentary water and snack when you board and I believe and can sometimes buy beverages from the carts. (This might depend on class of travel.)
If nothing else the La Feltrinelli bookstore in Florence station has good sandwiches and other premade train foods for sale. If you plan to arrive early for your train then you can spend some of that time in line at the bookstore bar. It sounds odd but check out the food in the cases before you decide not to. They'll even make you a fresh squeezed mixed fruit drink it you want. I would eat on the fast train rather than the local regionale as they are generally better set up for it.
Have a great trip,
=Tod