I made a maximum population guide for each of the planet types in the game to see how many people I could cram into a galaxy.


Interpretation Notes: If a race is not listed in the Best/Worst Race section of the chart, the race is at the median of all races. For Vents and Inferno biomes the median value is also the minimum value hence those do not have a worst race.

Analyzing from a Planet Perspective:
-Some planet types are good for most everyone and others are good for a few but terrible for everyone else. Paradise, Supercity and hive are good for most races (except water races). Ocean is pretty bad across the board except with water races where it's about average. Iceball planets suck for everyone except Orthin.
-The best terraformable planet for max population is Coral but only if you have Phidi/Thresher and Orthin pops. Otherwise, it sucks. Hive Planets are much better without those as the Metro biome is very consistent across races. Note that these are special planet types only available to Phidi and Ashdar empires respectively.
-Paradise has the best overall max population but it can't be created normally. I unfortunately have not seen a supercity planet yet so I can't tell you how they are created.
-Island is better than Garden with Phidi/Thresher and Orthin
-Garden is otherwise the best option for most races.
-Inferno is the worst planet type UNLESS you have Tarib in which case it is only second worst after Barren because Wrem populations kind of suck (each one is half a human pop and they grow slow).

Analyzing from a race perspective:
-Adding Phidi/Thresher and Orthin will improve the pop cap of most worlds due to their affinity with Ocean/Reefs and Vents respectively
-Adding Spice Mongers/Tinkers will improve any worlds with Arid biomes
-Adding Wrem to Iceball and Barren worlds and Tarib to Inferno will improve their capacities
-Yoral are probably the best race for colonization overall. They are above average everywhere but Vents/Ocean/Reef where they are average.
-Humans are interestingly average on pretty much all worlds making them like the Yoral but worse. If you look at the Best and Worst Races section, they don't appear much (above average on ice), meaning that they can fill in the gaps anywhere you don't have the optimal colonists.
-A lot of the best races in one biome are balanced by being the worst in others. Phidi/Thresher are great on Ocean/Reef but the worst on Airless/Arid/Forest/Metro
-In the games that I've played most worlds prior to terraforming are barren, iceball, inferno, and arid with arid being the most common of those. This makes Orthin, Tarib, Wrem(or Yoral), and SpiceMongers/Tinkers the priorities for population enhancement. After terraforming becomes common Phidi/Thresher are very important for maximizing the usage of Ocean/Reef biomes.

Note: The only reason to not include any particular race on a planet is if you have another race with more optimized stats and you don't want to waste the pop slot. However, this point is moot if including the non-optimized race would increase the available pop slots on the planet. Notably, Wrem are pretty terrible colonists with very low productivity scores and should never replace any other colonists. However, in the Wrem case, the Wrem have the highest population on the Airless biome. So, a barren planet would be most optimized by adding Yoral pops up to Yoral's max capacity and then "topping off" with Wrem pops. Same can be done for specialized planets. If you want to specialize an Ocean planet in Science, you can fill it as much as possible with Orthin and then top it off with Phidi or vice versa for trade or fill it with Algorians and top it off with both Phidi and Orthin for labor (I think Algorian had a +1 labor over baseline, they haven't been in any of my games).

Hopefully I've confused you all.