tax rate?

The way it works in GC2 is radically different than GC1, so don't try to make any comparisons or assumptions.

The class of the colony does not equal the maximum population. The theoretical max population depends on the class, but isn't linear (this much is actually the same as GC1).

As for Morale, there's a +10 bonus for an 11+ (12+?) PQ planet, but other than that, the PQ doesn't affect morale. (whereas with GC1, the morale hit due to population was heavily influenced by the PQ of the colony).

Personally, I try to stay away from having populations that high, it's too hard to keep the morale up. 13-15B per planet is my limit unless I have many morale resources and/or some morale bonus multipliers on the planet. Also, the later techs, and many of the trade goods will boost the population limit I'm willing to go for. Spending racial ability points on "Morale" helps too.

And finally, for the killer morale improvement, study "Xeno Ethics" and go Neutral. I play most games with morale/approval over 90% once I research that. Between the morale bonus and the automatic terraforming of all improvable tiles (even if you haven't researched the tech to do it), the Neutral option is VERY powerful.

Wow! 26.6 is a lot of population. Imagine a planet with 27 billion inhabitants.

The game can be played well with a much lower population. Don't build so many farms and don't use food bonus squares, at least until you have VR centers. Max population should not be a goal.

Population can boost your economy but it is a two edged sword. I leave all planets under 10 PQ at 5billion max population and usually don't increase over 10billion anywhere until late in the game.

I play as populist for moral boost and choose naturally joyous as an elective, I have few moral problems