Since I'm not familiar with FARs I don't know what they are referring to.
But you don't choose a critical engine, unless you are an aircraft designer. It is imposed on you.
And, as I have pointed out, jet aircraft do not have a critical engine. Nor do some propeller-driven aircraft.
For testing and certification purposes, where you do have a critical engine, however, you almost always assume the worst-case scenario, and have that one fail.
PS, No, there isn't yet a PPRuNe Safety Forum. It's on the way.