are there vans at the Florence airport that can accommodate 6 or more people?
They will accommodate you but you may have to wait for van
Be sure to go to official taxi rank out front
As you can see in the Google Maps images below (the first one is at the airport, the second one at the SMN train station) taxicabs in Florence are mostly station wagons but there are also occasional minivans. When you get in line they will direct you to the first vehicle appropriate for your party. Not sure if they'll be able to accommodate all of you, if there are too many of you and you have also a lot of luggage, but you can try. In any case the ride to the city center is about 30 euro (+ or - a couple of euro, depending if it's a Sunday or if after 10pm), so in case you need two cabs it shouldn't be more than 60 euro for both taxicabs.
You could use uber to request a van on the spot. However, most vans including taxi vans are pretty small. They may have 3 rows to hold 6-7 passengers but then no space for luggage. Roberto is correct to suggest using two regular cabs.
Another alternative is to contact a private transfer company to request a larger van to accommodate number of people and luggage. But these can be expensive.
Where are you going? There is the big Volainbus which can take you to the Santa Maria Novella train station.