Pvk: Did you ever experiment with using the "default population" size for colonies in the settings.txt file? (Don't have the game in front of me, but I think you can set a population for when a ship colonizes a planet.)
I've been experimenting with incorporating proportions-like settings into my own personal mod, though in a way that the AI can use w/o massive alterations. (Like creating "city" type Versions of various resource-producing facilities and making them available at higher levels of extraction so it becomes worth researching those techs past level 3.)
Basically, I've had troubles giving the colony modules higher cargo capacities than the standard cargo modules. Currently, 1M pop is set at 50kT, w/the cargo modules having multiples of that number in capacity. Would rather it be 500kT (did I mention I'm calling this Proportions-Lite?
) with growth every 5 turns. Just about ready to set colony module capacity to 0, set that settings.txt flag to 1M (unrealistic, I know) and making the cargo bay capacities divisible into 500.