Sneak Attack always stacks, so if you have the rogue talent, the bleed does apply, just add the sneak attack damage of the Magus to the Rogue one if you spend one point from the arcane pool.
Moreover, the bleed damage is increased because it relies on the number of D6 of the sneak attack
For example, let's say you are Rogue 3/Greensting Slayer 3 and you took Bleeding Attack as the Level 2 talent, when you are attacking and you can make a sneak attack, you have two choices:
1) You can not spend the arcane pool point for the turn and you just deal 2d6 from the rouge levels and apply 2 bleed damage
2) You can spend the arcane pool point and add the damage dice from the Greensting Slayer levels and you're gonna deal 4d6 sneak attack damage (2 from rogue and 2 from magus) and apply 4 bleed damage
It's kinda like how a sorcerer's bloodline arcana will effect all spells from all classes he has.