I build usually a starport on small planets, a factory/research building (depending if I go all-lab/all-fac) and if some space is left then this is a good place for a wonder.... maybe those that the AI usually doesn´t research, so the small planets has alot to do until all tiles are outbuilt.
Concerning the Econ-buildings: their benefit is applied on a %-rate, meaning, the higher the pop is the more $ will flow from one building; therefore their most efficient use is on worlds that are able to support a high population; eg. through moral tiles, moons, or first colony etc.... Small PQ 3+4 might not even raise 6mil inhabitants - allthough you can fly them in, and it is hard on small planets to keep moral high, I think there is an approval-boost applied when going beyond PQ +10 etc. But I´ve seen PQ +4 planets going to PQ +13 after researching Soil techs, so not all small planets might be that bad later on...
I think there is no static answer. Of course a planet´s output that has a Tech Cap will be mostly great when only Tech building are applied, (same goes for Manu Cap etc) but question is whether you have the economy input that is even able to support this output? And if not, Econ buildings might as well help on these planets, too....
Concerning manufacturing, try to match the output of each planet with the excact bc a ship will cost (or a division of that...) because any excess manufacturing is lost.... so its much better to overbuilt an Manu building with a Stock market, that way your Manu output will most likely be the same, but your $ income has increased....
Concerning Pol Cap - you can use it to sway moral in nearby worlds, and/or increase you influental range on sectors bordering alien races - this will boost your Tourism-income then; esp. on small maps to is a good factor in income.
As for the population growth/ approval-rate thing, well, I do it like this: keep it to 45% from the start; once the colony rush on my side is over change to 100% - as long as I can support this; then go back to 45% until all planets have maxed out population; and then going as low as you can, sometimes even below 30% - always keeping an eye on the Next Election-info, to raise the slider one day before to above 50%.
And for the starting-game bonuses, I have to say that Creativity can be helpful and Luck is very important if you are going to play a warfighting game. Luck determies from what point-on your dice roles, e.g. having Luck + Universalists will give half the defense etc even in the worst case. Hitpoints + Repair is pretty good if you are going to built big capital ships and try to keep them in order to be upgraded. Some ships I had had HP+400 and more, they are really worth the upgrade to better weapons systems then... Though most important in economics, without money you can´t do anything; although I'd like to know how these bonuses on Manu/Tech are applied: (1) Do they simply lower the cost needed for researching a Tech (or lower the cost a ship has) ?, (2) or do they act as a multiplier to actual pruduction? Because then, an actual bonus in these two fields is worthless if your econ is too weak to support is... or the other way round... if you have such a good economy that money is left then there is still the option to raise production by adding some planetary structures... So seen from that perspective Economy is most important, although if (1) is correct then there´s no difference.... Would be really interesting to find out, because that +20/+20 bonuses from the Mercantiles is heavy; +20 Research +1 Sensor is even heavier in terms of distribution points....