Hypothetically speaking if they were ever given electric pantograph cars, would 1 or 2 carriages be added?
It would appear that you need a transformer for every 5 coaches, so a 5 coach train needs 1 whilst a 6 coach train needs 2. 9 coach trains are fine with 2, whilst 12 need 3.
Therefore to lengthen a 4 coach train means the creation of a 5th coach.
However a 5 coach train would need 2 extra coaches to hold the two transformers.
That could get fairly expensive fairly quickly (what with ~40 X 5 coach units).
As such it could be cheaper to create 9 coach units by removing 1 existing coach from the 5 coach units to the replace with a transform. That releases 20 coaches from the existing fleet, these could be added to the upgraded units and a second transform be added to create 9 coach units.
They would be
5 coaches existing
2 coaches transformer
2 coaches from the rest of the fleet
That would give XC (if my maths is right) a single length of "short" trains (69*5 coaches) and some 9 coach units (7*9 coaches).
The 9 coach units would be of a comparable capacity to a pair of units. The problem would be that just for their fleet XC would have an order of 87 coaches, which isn't quite 100. 100 coaches is the approximate level where the cost per train falls to a fairly reasonable price per coach.
Repeat for the VWCT fleet, with the removed coaches moving to XC and XC could gain another 7*9 coach units (total of 14) at the cost of 7*5 coach units (total of 62). This would get us to 114 coaches.
This would compare with 126 new coaches but a mix of 5 and 7 coach units.