Of course underwing engines would not fit that, but do the technical arguements previously expressed in favour of underwing still apply to smaller airframes in theory?
Yes they do, but they are of course smaller in scale.
The main reason why fuselage or above wing mounted engines are used on smaller jets is the length of the landing gear. Imagine how big the landing gear would have to be with underwing engines, and compare that to the size of the plane. That would not only add weight that would outweigh the benefits of the lighter structure, but would also use a lot of space.