Would a sensible strategy from May not be to not pair up the 5s at all but instead make as many pairs as possible out of the 4s? They really ought to do everything possible to avoid sending 4s out on their own.
Also, if they could consistently decide which services would be doubles rather than singles, they could sort of manage the crowding by varying the prices of Advances to persuade people to use the doubles rather than the singles. This sort of thing is supposed to be an advantage of so-called "yield management", but CrossCountry seem to be entirely ignoring it. On flows where Advances are available (I tried York to Birmingham and Oxford to Manchester for example), look a few weeks from now and they're the same price every hour.
XC don't have enough Voyagers to make everything either 5 car or 4+4car sothere will be plenty of 4 car running.
From May (assuming all the services planned run) will require...
Hourly Edinburgh-Plymouth = 19
Hourly Manchester-Bournemouth = 11
Hourly Manchester-Bristol (+2xPaignton) = 8
The handfull of Reading to Newcastle services that run = 4
(Full hourly service would require 11)
That will require a minimum of 42 sets (less 2 that the HSTs currently cover).
I think 220019 is still long term out of service (damaged at Kenilworth quite a while back).
So a very rough calculation, of the 58 voayagers, a max of 51 sets avaliable for use daily, 1 is normally hot spare at New Street, so the most that can be 4+4 is around 10 diagrams, assuming XC fully diagram all the Voyagers. Obviously that will be less when the HSTs retire, and when Newcastle to Reading is restored to hourly.