Yep, the faster Reactors would require better Coolants to stay Cool, but an early Uranium Reactor could easily run using good old Steam if you dont fill it with too many Rods. As in real Life, you probably dont want Water based Reactors, at least Coolant wise that can end up horrible and also corrosive.
They can emit all the same Power for each Tier to not make it wasteful that way, you just need higher tier coolant for up-scaling the Reactors Main Power Output.
At least that’s what I think for now about it.