racial traits

The emotionless trait will override any happiness penalties/bonuses.

Space Empires Depot | SE:V Balance Mod

You can take Emotionless for 1000 pts, and then depressed for -500 pts. The result is that you get emotionless for 500 pts, because being emotionless, depression is meaningless. The way I look at it they are meant to go together, and equal 500. I have a race at the moment with both and it's fine. I expend fighters without a second thought and my people (well, technically my bugs) couldn't care less. Someone could come along and say that without depressed my emotionless might have a higher production bonus or something. I haven't really looked into it.

Your population has moods. Offhand I think they are rioting, unhappy, indifferent, happy, and jubilant. Each turn as good things happen their mood will go up. If depressed is -3% then their mood will go down by that amount each turn, apart from other modifiers. Rioting means your people stop contributing and I believe if they riot long enough they might secede. Whatever, they do less work. As they get happier you get a bonus to production.

The greatest benefit I see in emotionless is the lack of ingratitude in your population. Notrmally I could manage my people for ages, but just because I have the misfortune to lose a few ships to the enemy the ungrateful little proles start complaining. Sure I might miss overproduction because my population take no joy in their work but I prefer it. I don't always use emotionaless though. Just when it suits my race.

Emotionless is both a blessing and a curse so the 1000 seems about right when you cancel out 500 with depressed trait. Since although your planets will never riot and produce nothing, neither will they ever become jubilant and provide the bonuses that that brings.

BTW, how racial traits like unhappiness/ reproduction/ environmental tolerance affects alien (conquered or immigrated) races in an empire?

racial traits don't affect them.

your society and/or government does.