Use positive number for natural decrease setting in your happines type, let's say:
Natural Decrease := 30
This way your planets will quickly go mad, and you'll need to counter their separative mood with some happines improving facilities.
Edit: after posting this I've realized that it's not the solution of your problem: planet will decrease in happines even without Colonial Corporation.
Concerning Change Population Happiness - System modifier: only the best facility with modifier of that type will be taken into account. Unfortately, modifier amounts are not appled or substructed.
And since +50 is always better than -20, the only fist one will work.
Further edit
Try to use Change Population Happiness - System (for Colonial Corps) in conjuction with Planet - Change Population Happiness (for Colonial Authorities). This should work.
[ April 16, 2004, 22:16: Message edited by: aiken ]