I should think you could use the MT interiors as kitbash fodder for many of the other cars as well.
I would do as Nickleplate759 suggests.
The MTL interiors can be cut and pasted to yield the 14 and 16 section cars, and the 8-1-2s and 10-2.
They also can provide the 'sections' for other cars.
A source for more bedrooms and drawing rooms might be interiors from streamline cars, such as 4-4-2s.
If you end up with an open space here or there, lower the shades.