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Replying to my own thread.

If you have the Proliferation Tech researched, it looks like you'll have all available Alien Civilian Populations growing on your worlds at the same rate. So if I understand it correctly, it means that the Civilian Population will be growing at a rate
of 5 times as fast as normal. It seems they'll outpace your Imperial Population growth as well.
 
Additionally every race has their own 'compatibility' with other races enabling certain alien population limits to exceed that of their normal civilian limits (example being Liir civs living in the oceans when Humans live on land). This results in a high population max for certain 'compatible' combinations of Imperial Race -> Civilian Race.