So if I take AI and Inner Perfection, what should be my third civic?
In my last AI/IP game, I went with Technocracy. I actually started out with Technocracy first and added Agrarian Idyll as the third perk, which worked really well. Technocracy early means your early expansion hardly slows your tech at all. It also means that mid and late game, you don't really need many unity buildings if you go really heavy on research. Agrarian Idyll is quite nice as a third perk because the point where you get the third civic is around the time you'd be advancing your colonies past rural worlds anyway, so you get a huge boost in housing when you need it.
This setup allowed me to rush habitats, which house the entirety of my alloy and consumer goods production plus some of the rare resources needed by the upgraded buildings. Once I got my first ringworld segment, I reduced every other planet down to a mining colony so that all my food, energy, and research came from the ringworld. All my other colonies had stop growth, so their growth was directed to the new ringworld segments as I expanded it.
The ecumenopolis will do better than the habitats because they're still totally broken compared to anything else that does alloys/consumer goods/trade. But habitats are pretty solid, so you don't really need to restart over and over to get the ecumenopolis, unless you want to steamroll the AI even more...