[mst]

...and there was the generalisation that a FF sensor, no matter what, gains about 2 stops of light. I know it has to be sort of same generation sensors, but that wasn't said...

 

Come on, please! It was said, see above.

There is no camera which is the exactly scaled version of a smaller or bigger one. Therefore you have a nice formula, one just needs to build teh devices fullfilling it.

As is the case with any generalizing and thus idealizing rule. Yet, it's still a rule, and it's up to you to derive decisions from it and apply them to a world that is more complicated than the rules assume.