Stack Benefits

When a class has an ability that mimic a proficiency can they take the proficiency and stack the benefits? Bladedancers can do this with Swashbuckler…what about something like Mystic Aura?

If the class can take the proficiency to stack the benefits, it will be listed in its class list.
For example, Swashbuckler is a Bladedancer proficiency, so Bladedancers can stack Swashbuckler on top of their inherent AC bonus.
However, if the proficiency is NOT listed on the class list, then the class cannot stack.

So what about custom classes and the various profs? Can they/should they stack things? Or more precisely what shouldn’t stack?

Unfortunately, the answer to that is “therein lies the art of building classes”. There are no hard-and-fast rules for which proficiencies to assign to which classes.
If you review the class lists you’ll see that all of them have a mix of special, sexy proficiencies and general, professional proficiencies in their list. All of them are missing some proficiencies that would be helpful, but which, for whatever reason, weren’t included.
As a rule of thumb, I’ve allowed:

  1. Stacking of an attack bonus with stacking from fighting style for one weapon type
  2. Stacking of surprise roll modifiers
  3. Stacking of initiative adjustments
  4. Stacking of ac bonuses for classes that wear leather or no armor
    I’ve avoided:
  5. Stacking of multiple proficiency throw bonuses (e.g. Trap Finding x2)
  6. Stacking of ac bonuses for classes in chain or greater
  7. Stacking of damage bonuses outside of fighter damage bonuses
  8. Stacking of “chance of success increases per level” benefits in such a way as to accelerate the curve