While tiered reactors and reactor components fit GT6 perfectly, I think tiering the coolants and fuels isn’t such a great idea, as it would reduce the number of viable reactor designs.
Tiering could however be done in a non-linear way with different fuel and coolant tiering lines, like a water, coolant, salt, gas and liquid metal, with their own progression and the lines all being equally viable (with different use cases ofc). Water to heavy water (maybe even pressurized water and heavy water, but that would need changes to the piping mechanic), mercury to lead to sodium-potassium alloy, etc.