Well, I think that Spearmen, Swordsmen, Assassins are probably mercenaries because they are humans and they like to fight. Shadowlings and Spriggats on the other hand might be slaves because of the knowledge they have. I doubt that a Spriggat would want to fight, more or less a shadowling would want to watch the “fleshings” battle it out. Archers and Healer and Spirits, I think could go both ways as they could have equal reasons not to fight or to fight. I think Healers would be more inclined to be slaves because (and even though they aren’t all like set) they wouldn’t want to hurt people.
But Hey a Character is a Character