Q 506

If I make a Rogue that takes either the Knife Master Archetype or the Vicious Claws talent, then takes levels in another class that grants Sneak Attack dice such as Assassin, can the SA dice from that second class be rolled as d8s provided I'm using the right sort of weapon?

For example, say I'm a Catfolk Rogue 5/Assassin 5 with Cat's Claws and Vicious Claws, would I roll 1d4(claws)+3d6(Assassin)+3d8(Rogue) or just 1d4+6d8? I'm leaning towards the latter, but I'd like to know for sure since my DM will probably have a problem with this, though I can't see why.