I stopped playing because of the lags somewhere around 2400. But even up to this point, most of my planets are not completly full. Population is a really important resource. I would not sell them. I played as slaving Xenohaters, but the first thing i do after i occupy a planet is declaring them as non-slaves, making them "Resistence?" (Forgot, how to spell it properly). Yes, they need more resources then. But only for a short time, until they calmed down and i am able to get into surplus again. And then everything is nice. Checking out my empire(?), i have about 2500 population, about 600 are from my starting race. The rest is from the occupied empires. And still, most of my planets have space left for settling there. If they want.
I learned, that the races (? strange word) preferr their favoured planet system. When settling on new planets, which are not my favorite ones, only the leaders and experts are from the starting race. The rest fills itsself up from all the other races, which does prefer this planet by their preference. I wanted to try out, what happens after terraforming, but i had to stop the game due to this crazy lag times, where you have like 2-5 sec for just one day...
Ah yes, somebody was writing about assimilation: this is strange, as it does only change the chosen race a bit. In my example, i had psionic powers. The "assimilated" races got this trait and then went back into slavery again. I had to put them into "non-slavery" manually again.