Mixing Caster Classes.

Yes if you mix caster classes they wont stack (the only exception apparently being Fluder but that is still kinda a mystery of how he did it, maybe a new world exclusive Ritual? Who knows). So yes, if just like your example if you are a level 63 Spiritual Caster and then you drop the rest of your levels in Arcane caster classes they wont stack and you won't be able to use 10th tier magic.

The question about Super Tier Magic I cannot answer because we only know that you get access to it at lvl 71 (with a caster build because it would not make sense otherwise) but if they need to stack to gain Super Tier Magic is unknown, my personal bet would be yes but again we have no 100% clear evidence.

(Other members of this sub please correct me if I forgot something or remembered something wrong.)

/r/overlord Thread