I think it is pretty clear you could make it fit under a 20m car. Whether the full new design would be worth making, when considering against an existing design such as the FLIRT will of course depend on other needs.
It's of note that the 755/231/756s were not ordered for routes that are restricted to 20m vehicles, and that is because these units have quite a lot going for them in other areas, such as level boarding, bimode/trimode capability, and easy potential for future upgrades/replacement of power source, among others
Thats quite a difference (noting it’s a low v high density comparison). And the 755 is a fixed 80m - which would look a bit silly on the Looe branch in winter!
The 755s are truly a low density set-up, aiming for comfort. 2+2 seats that match SWR first class seats, 2 wheelchair spaces, 2 toilets, and 6 bike spaces. if needed for GWR/northern/whoever else, a much higher density interior could be used
Also note I cited the seat numbers for the GWR 150s, which are the highest-density ones. Northern has some 150s with only 124 seats per unit/248 for an 80m pair