Just uh, three short house rules I use in homegames for stacking archetypes that I wanted some feedback on:
-Two (or more) Archetypes that alter the same class feature but in different ways are stackable.
So, for instance, the Shadow Caller summoner changes the class' Summon Monster list and changes your Eidolon's appearance while the Master Summoner changes the Eidolon's stats and how often you can use the Summon Monster SLA. Despite changing the same two class features, these two archetypes would be compatible.
-Archetypes that replace an ability with a functionally identical version of the same ability can be traded out again.
The best example I have here is third party: The Dreamscarred Press Path of War Stalker gets to add bonus damage from its Deadly Strike class feature to the next attack it makes after it recovers maneuvers. The Vigilante Stalker archetype instead adds sneak attack damage, because the archetype trades deadly strikes for sneak attack. Otherwise the features are identical. So I'd let the DSP Vigilante pick another archetype that also changes maneuver recovery.
Because a lot of the time archetypes will have to change minor things simply to remain internally consistent, but often those changes aren't meaningful enough to necessitate exclusivity in my mind.
-If two archetypes would otherwise be compatible, but both replace a class' 20th level ability, you may stack the archetypes and disregard one of the capstone changes.
Basically the number of games I've seen that reach level 20 is very rare. The number that stay at level 20 long enough for capstones to be relevant or significant is even rarer, so I think it's a shame to lose out on potentially cool archetype combinations because of a conflict that will almost never be relevant in a typical game.