no haha feel free to devise your own creature if you so desire. so long as it makes sense in the ivalice alliance universe (which, if we're being honest, what doesn't lol) you can go wild!! of the four you listed, though, i think behemoths have the most probability of being too much for most clans to handle on their own?
apologies that i didnt see this sooner, oops. nothing arcane just yet, otherwise its free game! other clans are always a good plan, or a simple mark hunt (that is to say, someone puts a bounty on a monster's head, probably the local ruling lord or something). considering labanar's status, though, it might be wise to use something inordinately powerful, yeah?
Iron Knights are definitely a physical class, but that doesn't necessarily mean they can't use magical effects; rather, they can't produce them. If you could get your hands on a vial of fiery-something-or-other, then elemental sword skills are entirely within their boundaries. Or the swords themselves could be perennially on fire, go wild (if so, make sure you mention it in the equipment section though).
I'm allowing a pretty huge creative license in terms of the hybrid classes (of which the Samurai is kinda one); as long as it's within the boundaries of common sense and the class's respective lores, feel free to do whatever feels right for your character haha