It occurs to me that its possible to trade gold for resources. This is not something i have done so far, but assuming it doesnt cost stupid amounts, it might just be possible to run a 3 city empire.
First city: Production for key wonders. We need the pyramids (for the 3 happy faces, and because i like the damn thing) and whatsitcalled that gives all religion civics for free religion. This will keep people happy and no religious tensions with the other civs.
Second city: Gold city. This city will generate a GS first (we only really need one, at least in the beginning). After that it will focus exclusively on GM who will be settled in the city. With the gold we can buy health resources, and the extra food just makes it that much easier to run specialists.
Once enough GM have been settled to pay for all the resources the other civs have, plus a little surplus so we can keep the science slider maxed, it will switch to science production. Maybe make another GS and then farm cottages.
The third city is of course science city. A big well developed city can pretty much keep up with an entire AI civilization, at least on noble where i play.
There is still the option to go to war with this setup. We dont need every single wonder, just the important ones. The rest of the time we can build whatever we want.