This is correct, although the limitations would be Maiden Newton and Frome at both 4 coaches maximum, additionally Castle Cary platform 3. Bruton will accommodate 5 coaches.
On a 5 or 6 car on the Heart of Wessex, 99% of the time you'll find the rear unit locked out (unless it is exceptionally busy where the front 4 coaches may be used if Castle Cary platform 3 isn't involved). For 2O84, I am very confident come the time, it will only be 3 coaches for the simple reason that the rear unit won't be used for passengers. 2V82 is actually booked a 6 car, with the rear 3 locked out.