I believe you are mistaken here...? You get a set amount of gearsets when they're first unlocked, and each time you unlock a class or job after that you get one new gearset slot. I can't remember how many you get by default, but the end result is that you end up with like 5 or so extra gearset slots even if you make one for each class and job in the game. I have all my classes and jobs in gearsets no problem.