The ratios don't get halved as well do they? Otherwise Later Romans with 0-1 Ps(O) per Ax(S) would only get 0-1 Ps(O) per 2 Ax(S), changing the character of the army.
I though the minimum number of Blade was 16 anyway, so would become 8, which isn't a problem.
If it were 8, which would halve to 4, and the ratio was 1 per 8-12 Blade then I guess you would be stuck with a minimum of 1 Cv(O) (kind interpretation) or a minimum of 8 Blade (less kind interpretation). In a Polybian Roman army this seems appropriate anyway - you should have a lot of Blades.
Toby