As far as I understand ship blueprints (didn't mod them yet) there is no "BestOffense", just "BeamWeapon", "MissileWeapon" and "KineticWeapon" what presumably is the respective best weapon of that type.
SInce in Crusade every single weapon is supposed to fire individually (couldn't check that out yet, no war for me so far
) I agree that we should have more fine tune possibilities for modding ship blueprints. Since the AI is also using the blueprints they would also not be able to use the finer details of mixing different weapon strengths of the same type.