I love newbies!

[Jiramisu]

The game encourages following multiple classes on a character. You have to get at least two of them to unlock the advanced jobs anyway, and you can use certain skills from a class on other classes you advance with the same character.
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Awesome, thanks for the info! Makes me feel good about going Gladiator then, sounds like a great balance between switching off between Conjurer, Marauder, and Gladiator which are the three I wanted to try the most. Guess I need to familiarize myself with the jobs more to know where all the mixing occurs.

[Niwashi]

Guess I need to familiarize myself with the jobs more to know where all the mixing occurs.

To start you off, here's a list of the jobs and which classes they require (or can use).

The class listed as "primary" for the given job is the one the job specializes. For instance, a Warrior is really a specialized type of Marauder. It uses Marauder weapons and has access to all of the regular Marauder skills and traits, but then adds a few specialized Warrior skills as well. A job also shares the same XP and level as it's primary class. The primary class for each job has to be at level 30 (and have completed the level 30 class quest) in order to unlock the job.

The class listed as "secondary" has to be at least level 15 to unlock the job. The one listed as "tertiary" isn't actually required, but certain skills from both secondary and tertiary classes are available to use when the job is active. (For instance, while it's not strictly necessary to level Thaumaturge in order to become a White Mage, White Mages are going to want access to the THM skills "Surecast" and "Swiftcast", so they need that third class in order to be more effective.)

Warrior: Marauder (primary) + Gladiator (secondary) + Pugilist (tertiary)
Paladin: Gladiator (primary) + Conjurer (secondary) + Marauder (tertiary)
Monk: Pugilist (primary) + Lancer (secondary) + Marauder (tertiary)
Dragoon: Lancer (primary) + Marauder (secondary) + Pugilist (tertiary)
Bard: Archer (primary) + Pugilist (secondary) + Lancer (tertiary)
White Mage: Conjurer (primary) + Arcanist (secondary) + Thaumaturge (tertiary)
Black Mage: Thaumaturge (primary) + Archer (secondary) + Arcanist (tertiary)
Summoner: Arcanist (primary) + Thaumaturge (secondary) + Archer (tertiary)
Scholar: Arcanist (primary) + Conjurer (secondary) + Thaumaturge (tertiary)

[Sagittarian]

Check out here for a list of the available cross-class skills which may give you an idea of how they handle.

From my perspective for the tanks:
For 8 person stuff, gld/pld makes the best main tank and mrd/war the best off tank.
The gld/pld generally takes a beating better. Most of their skills are focused on defense plus they can stoneskin themselves if the healers are busy. Their best use as an offtank is to cast stoneskin on the main tank when needed.
Mrd/war has some combo skills that debuff and can do a good amount of damage when not focused on defense. They have more hit points than gld/pld but take more damage. Some healers prefer dealing with them, some don't.
At low levels in 4-person content mrd/war has better aggro control with overpower but gld/pld evens it out at level 40 with shield oath.