In Fxcom you usually build up an A-Team of supersoldiers, and use them all the time. WoTC add-on tried to fix that, but it didn’t succeed. However, there is a second, less popular and less efficient, but more interesting (for some) way to play - make A-Team, B-Team and even sometimes C-Team and rotate them with every following mission. But yes, you can’t take more than 6 soldiers with you on a mission.
However, in PP you can take up to 16 soldiers. If they are all A-Team superheroes (like in FXcom), this will just ruin the balance, making hardest missions on hardest difficulty walking in the park. So, as far as I remember from what Julian said - you may go that way (build a supersoldiers team), but it is unlikely you can succeed training too many of them. They will be few in number, and probably very expensive to equip and train. However, there will be another option to take a lot of weak (or “normal”) solders on every mission. Like it was in classic Xcoms. Even if you have 3 transports, you still can send 3 x 16 soldiers at once on different missions, simultaneously. Or, if you prefer FXcom way to play, send some 4-5 powerful “heroes” on each of these missions, again, simultaneously.
In the first case you ll have a pool of ~50 soldiers. In the second, of ~15 powerful soldiers. But if you limit cargo capacity by the number of soldiers taken, 15 powerful soldiers pool is the only way to go, it will be too efficient to ignore.