Just a thought -- why not allow more than one extreme colonization property on planets? That is -- why not have "aquatic toxic" planets and "barren heavy-gravity" planets, etc? 50% production (and colonization, for that matter) would require all the pre-req techs, and so on for 100%. It could also make the spore-ships make a bit more sense -- instead of changing the planet to a toxic one, overriding a previous property, it could simply *add* the toxic property.

Anywho, just a thought.