
Your suggested rule for a fast-acting poison seems adequate. Of course, it is hard to say in theory; running a few tests and putting it into practice would be required to find a nice balance between "Ineffective" and "Overpowered."
As things are, I wonder if the poison is not too powerful, as it would knock out pretty much any character in a matter of rounds, what's with the damage being so brutal and the Difficulty of the Stamina checks being so high... But then again, that's what such a poison is made for!
Again, logic dictates that such a substance would be hard to find and expensive, since only assassins and the like would use it. Probably several azure daols for a simple dose!