Additional question to the above questions...
In the second paragraph to the list notes, on line 15, it says "Regulars must be in the C-in-C's command."
If this comment relates to the Britannia Prima/Riothamus option, it's irrelevant as those list options already all read "Replace C-in-C's X with Y", meaning the quoted statement above is redundant.
But if it relates to the list in general, does that mean the Britain before 429AD options are only applicable to the C-in-C's command - that is, that the Alae, Equites Dalmatae, Legionaries and Auxiliary cohorts can only be in the C-in-C's command? This is particularly relevant to the Alae, which (as mentioned in the OP) appear to be required for all commands, without the ally-generals themselves also being upgraded.