Well, I want to be sure that I'am right because I'm in the middle of something really great...if it works the way I think.I think the way it works is mostly as you say, the creature generates poison, if he is in the same space as the boss then BOTH of them get poison tokens, and at the start of each creature activation that creature takes the poison damage - so they will build up each turn. Pretty nasty if the boss doesn't have any way to move...
So the Orc Boss Hekia in his tower, is near another creature, with fire wall 1 and poison cloud 1...the fire doesn't bother him, but the poison does as he shares the same space as the Boss.
The creature spawning the poison activates first, so each turn, I put a strong poison token on the boss and he is wounded from the old token and the new one...is this correct ? if yes, I can let my party watch him dying in the green cloud alone in his tower.
(I might be misremembering here but doesn't Poison Cloud 1 put poison tokens on adjacent diagonals to the creature? If so then it won't generate poison on itself or the boss)