S
Sunseeker
Guest
Not all of them are. Look at Warlock or Paladin subclasses, you'd have some difficulty having more than one of each of those simultaneously.
As always, I'd put it on the player's head to give good reason why their character is X and Y. I'm fairly strict about multiclassing because I rarely find people do it because "Wouldn't it be cool to be a barbarian monk!" and more do it because "Man if I combine these two cool abilities I'll totally rock the damage charts!" And I like to keep a lid on the latter. The more extreme the combination, the more burden the player has to explain themselves.