Hi!
It seems you cannot use the Aim action with the free attack during a Sneak Attack or Ambush with a range weapon which seems a bit odd to me. At least in an Ambush when you are not moving you should be able to Aim.
So in my games when you successfully Ambush with ranged weapons you get a +2 modifier to your ranged free attack. This is a special case that overrides the rule that you can do a slow OR fast action (in this special case it is both).
Thoughts?
-Simo