Mages can cast a spell that'd cause them to get more than 200 fatigue. Mages can't kill themselves with one casting. You can cast even Army of Lead (several hundred points of fatigue) with caster without any extra levels or gems, and he won't get more than 200 points of fatigue (and thus no damage).
I've found that in my use of Blood and Air magic, thats not quite true. Mages will very often "go off script" and not cast spells with more that 100 fatigue, especially if they have any fatigue.