Unlocking paladin will require you to have Level 30 gladiator and level 15 marauder iirc, so you'll definitely want to use some of them. But yeah, every class will see substantial benefits from even just levelling other classes to 10ish for additional abilities. In particular a Gladiator would also benefit from the Pugilist abilities that give you an evasion bonus and the "counter attack" that slows enemies when you dodge/parry I'd think, and both of those are available very early.
It's 30 gladiator and 15 conjurer.
You still need to level marauder to 26 though to get all their useful cross skills. Conjurer not so much, really up to you if you want to take them all the way to 36 just for stoneskin.