If I take a 1 level dip into base rogue, and 19 levels of slayer, I end up with 7d6 sneak attack (at like CL 19 I think).
What if I take a dip into rogue with the Knife Master archetype? Does it modify ALL my sneak attack, so that as a R(KM)1/Sl 19 I would have 7d8 with knife master weapons and 7d4 with anything else?
Yes. If a class feature modifies an ability, it modifies it for all classes that grants that ability. For example a sorcerer bloodline arcana that modifies spells modifies all spells cast, not just sorcerer spells.
