I'd just run them at their nominal voltage, so 3s for the 12v ones and 5s for the 18v ones. The reliability will be greatly reduced if you try and run 12v ones on 5s, that is for sure. You're more than doubling the torque and putting half the RPM again through a gearbox that isn't designed to take it. This is why Tormenta 2 has so many breakages, we're way beyond the manufacturer's spec. Plus any overvolting is likely to (and rightly) make Ewan less interested in trying to help if something blows.

12v on 4s would probably be alright, but again, you will have absolutely ample power if you run them at their nominal voltages, and very respectable speeds if you opt for 24:1 gearboxes rather than 36:1.

Perhaps try to focus less on the details (you will hardly feel the differences in the arena, if at all, is the truth) and focus more on design. Cobalt mk1 was the perfect example of having just two 550 motors outclass most of its often more powerful rivals, and that was down to good design.