G’raha’s unique quality of being versed in three weapons styles (gladiator, thaumaturge, and conjurer) is mostly explained in game by the fact he’s actually several centuries old and not because of his usage of job stones, like us. His role as a trust unit is just to fill in any role we need for that instance, thus his use of the default tank, caster, and healer options. I don’t think the devs would give him a class locked entirely to him since they need to recycle resources for characters used heavily. If G’raha got a unique combat style, it would be an enormous amount of work for something rather small in the grand scheme.

Meanwhile, Firion’s many weapon conbat style is actually a callback to how combat and experience worked in FF2, which was based on, if I recall, swapping around everyone’s gear to raise certain stats bit by bit. (They never used that set up again because of how cumbersome it was!)