Chlorine makes 0 sense for a life-supporting atmosphere. It is too reactive of an element and tends to destroy any molecules it comes in contact with. Complex, sentient lifeforms evolving on a chlorine atmosphere world would be too unbelievable.
Argon only makes sense if you pretend the race doesn't breathe anything and just needs pressure.
What would be best would be the ability to mod in new atmosphere types and to mod which can be selected by a race for a homeworld type. Then, you could have Chlorine atmosphere planets, but no Chlorine-breathers. You could have Sulfur Dioxide atmosphere planets (think volcano worlds) and no native organisms possible. That would be cool.
It would also force the game to have planets that you can not optimize by adding some new people to it.
[ February 15, 2003, 23:58: Message edited by: Imperator Fyron ]