yes, i am sure it says in the description of hide somewhere.
Yes they stack, which is especially good for PvP zone as people with +Perception powers wont be able to spot you unless they stack.
I was wonering this too, as it says in the stealth description that stealth doesnt stack with anything else. They do stack though, or I'd not have been able to stand inside the Bloody Bay heroe's entry chopper saying spooky things in local chat...
What the subjects says.