Hmm. If you are planning on having the computer handle migration for you ala SE5, might I suggest Target Population Densities. It would just be slider bars for each valid race that you have access too. Don't want any aliens on the homeworld? Move the slider bar for your race up to 100%.
That brings up another question. How does SL handle non-breathable atmospheres? I'm thinking of a system where non-breathers suffer infrastructure limits. So you have 500M non breathers and 100M breathers on a planet? The 500M non-breathers would suffer an infrastructure limit (based on planet size), while the 100M breathers would work "outside the dome" and have no such limit.
So, if the non-breather infrastructure limit for the above planet, is, say 250M, then half your non-breather workforce would be without infrastructure. So you'd have 350M + (250M*10%=25M) = 375M production (assuming there was enough infrastructure in place for the the 250M non-breathers + 100M breathers).
Another thing to consider would be how having 500M non-breathers crammed into a 250M rated "dome" would affect happiness levels.
