You've probably worked out the general principle already, from the examples in Growing your population, Feeding your people and Money matters. This page tries to pull these ideas together and give plenty of examples of what types of colonies should specialize in what.
The benefits of specialization are:
- You maximize industrial, research and food output by doing them in the most suitable colonies.
- You minimize building maintenance costs and building time by not constructing buildings that do things for which colonies are not suitable.
In the earliest part of a game you have no option — only your homeworld has enough population to build ships. But as the game goes on, you should aim to do most of your shipbuilding at Medium or larger planets that are Rich or Ultra-rich and have large populations. If you don't have any of these:
- Terraform sub-Terran Medium or larger planets that are Rich or Ultra-rich to increase their population capacity.
- Meanwhile build ships at Large or Huge planets whose mineral rating is Abundant (i.e. average).
Medium or larger planets that are Rich or Ultra-rich and have large populations may be as good at farming or research as e.g. your homeworld, but the advantages of concentrating shipbuilding at the industrial monsters are:
- You get faster ship production that way, while other planets can do research and/or farming just as well.
- You probably reduce building maintenance costs by constructing the top-end construction and pollution-control buildings only at your shipyard planets.
Sometimes the colonies that are most suitable for shipbuilding are in the wrong places, a long way from the fighting — this is most likely to happen in the middle of an invasion campaign, as you invade further and further away from your home region. The ideal solution is to research faster drives: the upgrades are free and immediate, and your warships will also benefit from increased maneuverability in combat. There are also 2 technologies that increase the speed of ship movement between your colonies, Jump Gates (a mid-level tech) and Stargates (top-level).
At your most fertile planets, of course, and those with Natives — even in Desert or Tundra environments Natives are more efficient farmers than your race is on Terran planets, and they can't be used for any other purpose.
- Your empire will need fewer farmers to produce the same total food, so you'll have more people available for industry and research.
- You may reduce building maintenance costs by constructing the top-end farming-related buildings only at your fertile planets.
Farming is a less clear-cut case than shipbuilding:
- You need to keep an eye on the costs of Freighter operations.
- Sometimes you may run short of Freighters, e.g. if you've just conquered a no-food planet with a huge population.
- You'll terraform some planets to increase their population capacity, and that will often make them more fertile.
Artifacts planets are best, even if they have relatively small populations:
- Those with high populations should go all out for research, including all the research buildings and morale boosters.
- Low-population Artifacts planets still give a bonus of 2 RP per scientist, rather like a Supercomputer but without the maintenance cost.
- If an Artifacts planet can easily be terraformed to give a large increase in population capacity, do it and then build the research and morale buildings.
Other planets with fairly small populations and below-average fertility should also concentrate on research, unless you're terraforming them or using them as as Housing colonies.
Since Housing works best for colonies with a current population of 1, colonies that you don't intend to grow are an ideal place — for example a small, unpleasant rock that you colonized only because it has gem deposits worth 10 BC per turn. But the rock must have a maximum population for your race of at least 2 — Housing won't work if there's no room for the extra colonist it produces.
Every colony with 4-5 citizens (excluding Natives and Androids) should build a Spaceport as soon as possible if you researched that tech (there are alternatives at the same level). Most colonies of that size or larger should build a Stock Exchange when you research it — it's the only Tech at that level, so even Uncreatives can always research it. The only good reason for not building a Stock Exchange immediately is that you seriously want to build something else first. But then build the Stock Exchange as soon as there's nothing else to build — if that leaves your production stockpile low when you finish researching something very desirable or you unexpectedly get the tech by conquest, spying or trade, the money produced by the Stock Exchange will help you to buy it when half-built.
The other most productive source of income is colonies with gold or gem deposits (5 BC or 10 BC per turn respectively, irrespective of the colony's population). Sometimes these deposits are on good planets, which are often guarded by Space Monsters, but sometimes they're on otherwise undesirable planets — the worst case is Tiny Toxic Ultra-poor Low-G). Don't bother developing the undesirable planets; use those with a population capacity of more than 1 as "baby factories" (Housing only works on planets with spare room), with at most an Automated Factory — if that causes pollution, don't build pollution-control buildings. If all your good planets become full, make the undesirable planets with gold / gem deposits produce Trade Goods instead of Housing; and produce Trade Goods at any gold / gems planets with a population capacity of 1 — even if they're not Toxic, it's still not worth terraformimg them.