Stacking size damages

Question

My question is mainly concerning Eidolons and damage stacking abilities.

An eidolon has an evolution called Improved damage that states it increases the die rolls for a specific attack by one step.

One of the eidolon’s natural attacks is particularly deadly. Select one natural attack form and increase the damage die type by one step.

Along with that there is a feat of a similar name called Improved natural damage that does the same thing, of increasing damage by one step.

Choose one of the creature's natural attack forms (not an unarmed strike). The damage for this natural attack increases by one step on the following list, as if the creature's size had increased by one category. Damage dice increase as follows: 1d2, 1d3, 1d4, 1d6, 1d8, 2d6, 3d6, 4d6, 6d6, 8d6, 12d6.

Lastly an amulet of mighty fists can get the +2 enchantment of impact which also does a similar effect.

This special ability can only be placed on melee weapons that are not light weapons. An impact weapon delivers a potent kinetic jolt when it strikes, dealing damage as if the weapon were one size category larger.

Legally would all these stack? And if so would they also stack with the eidolon evolutions of making the creature large/Huge and/or using enlarge person on it?

Answer 1

From the FAQ (i.e. official errata):

As per the rules on size changes, size changes do not stack, so if you have multiple size changing effects (for instance an effect that increases your size by one step and another that increases your size by two steps), only the largest applies. The same is true of effective size increases (which includes "deal damage as if they were one size category larger than they actually are," "your damage die type increases by one step," and similar language). They don't stack with each other, just take the biggest one. However, you can have one of each and they do work together (for example, enlarge person increasing your actual size to Large and a bashing shield increasing your shield's effective size by two steps, for a total of 2d6 damage).

So Improved Natural Attack feat, Improved Damage evolution, and Impact amulet don't stack with each other. Large evolution and Enlarge Person don't stack with each other. But you can take one from the first category and one from the second.

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Now, despite what the rule actually says, it's possible that it wasn't intended to work that way. Prior to this FAQ, the rule was "Multiple magical effects that increase size do not stack." (quoted from several size-affecting spells.) Under that old rule all of your mentioned abilities would stack with each other because most of them are non-magical. The FAQ both added the clause about effective-size-increases (which is clearly the main point of the FAQ), and removed the "magical" qualifier. I'm not sure whether the latter change was intentional, or just sloppy wording from the FAQ author.