[Windborne]

If neither race can breath the atmosphere type it doesn't matter which race has the most population on the planet, you can mix races on a single planet, and I enjoy doing so.

The benefit of mixed races it that if you use the "Upload cargo" button you can choose which racial to put on a colony ship, plus aiens tend to revolt less if mixed with a good dose of your own people.

You can tell what race(S) have colonized a planet by way of small racial pictures located with the planetary information, if you right click on these pictures you can even call up a screen that lets you access information on each of those races.

Remember though, if you want to stay out of domes you MUST have the larger population able to breath the atmosphere, world type doesn't matter here, nor does the fact that the less populous people have to live in domes or use rebreathers or something. lol