For the smallest worlds, the basic general minimum is as follows:
1 morale building
1 food building
The end. Two tiles.
For planets with a few more tiles, the profile shifts more towards:
We're still talking tiny planets here.
For anything larger you will want a manufacturing building so that you can build/upgrade more stuff in a timely fashion (a research planet is taken out of commission for a LONG time when you get a tech to upgrade your research buildings unless you have a MFR building to help upgrade those):
Beyond that you just increase the # of food/morale/spec buildings to scale to the size.
In the midgame you tend to need some morale pumps. Maybe up to 4 morale buildings or something. Lategame and with some of the economic starbase upgrades and relic artifacts, you can get some insane passive morale boosts, so you can often drop some morale buildings at that point (one game I had something like 65 approval on a planet with 0 morale buildings, and each morale building added like 25-30 morale, it was stupid).
> Do you specialize heavily?
Basically yes, although if I were to play a giant galaxy I would have a lot more slightly diversified/hybrid planets controlled by the governance wheel.
> How much production do you aim for on non-prod. worlds?
1 Quantum Power Plant on a tile that has a MFR bonus or if nothing else then attached to the capitol tile. As the other poster noted, most stuff is in multiples of 15 so early game aim for 15/30 as breakpoints for minimum social manufacturing thresholds. Mid and lategame it's kind of impossible to hit less than 15/30 social production at 100% soc mfr. In that case, when upgrading, you just throttle down soc mfr down to 15/30/45/whatever.
> Do you have a rough ratio for science/wealth/prod worlds?
Nothing I've done a lot of analysis on. I did some very casual analysis and "food=spec" works reasonably well for optimization at certain tech levels with certain assumed bonuses. As a result I run roughly "specialization = food buildings" as a generic rule of thumb, but a lot of my decision making comes down to available tiles and how they are spaced. E.g. if there's a cluster of ~5 tiles in one spot and a cluster of 10 in another spot on the planet, I will use the 10 for specialization builds and make the planet some absurd capitol (like research capitol with the galaxy-unique building etc), and spend the 5 on food. Spend a loose tile on MFR and spend another loose tile or three on morale.
If you want a planet to be somewhat hybrid, then you want to emphasize food (population = production for all 3 categories) with ~2 spec buildings of each type. Maybe something like 8:2:2:2:2 food:mfr:res:wealth:morale.
Please note that a game/race where you already plan on going heavy hybrid with this strategy means that you have high base production (high food) but you have comparatively low multipiers (your mfr/res/wealth multipliers will be like 150% or 200%, as opposed to a specialized planet's 900%). This means that racial +% bonuses or starbase +% bonuses or artifact +%bonuses ... basically any mods that are outside of tiles on your planet become very important.
If you specialize all your planets heavily, then racial bonuses like +20% research are worthless. +20% when your research planet has 1200% research? Yeah whatever. +20% research when you have 10 hybrid planets that each have 150% research? Ok that's a bit more relevant.
Same deal with artifacts or certain technologies. For example tech specializations that give "+10% research" -- same deal as above. When my research superplanet has 1200% research, that 10% means zilch. On the other hand, in a hybrid empire approach, that 10% can add up quickly.
> Do you have small worlds go all production since they can't do anything else anyway,
Absolutely not. Manufacturing is the worst because you spend MFR to build ships, aka military. Now you have to switch this random worthless planet's investment bar from social to mil all the damn time. So obnoxious.
For small planets I always make them wealth or research (or military building complex if they're large enough), and let them go afk and unattended for long periods of time.
When specializing, I always make large planets into manufacturing specs -- I don't want to have to micro many planets, and MFR requires the most micro due to the shipbuilding switch offs -- at least the way that I play. I can imagine it makes more sense if you constantly pump out ships -- in that case you won't have to go back and fiddle with your planet that often.