An argument for the pace and not the cubit

[Macedon]

I have already answered that. Polybius wrote in Greek for mostly Greek readers. These terms are used in the context of rank and file, so a "quinqunx" formed Roman phalanx would be a peculiarity he would have been forced to describe. Such a formation is described in Greek manuals as one of the cavalry rhombus formations (called "by rank but not by file"), so it is not that Polybius lacked the understanding or terminology to make such a description. You also have to account for his assertion that the men behind (these he does not call epistatae but ephestotes, which is a term independent from rank and file meaning those on the rear in general) were not able to support the first rankers, either with their swords or by "pushing" against them. A formation like that you (and Mark) propose would have the second rank (in a formation by rank but not by file) actually support the first rankers and this is why you think they should be posted there. IF the Romans fought in that way and Polybius did not describe it, when he is asserting to be pinpointing the differences between the two modes of fighting, then his credibility as a source suffers greatly and so, the whole argument demands that Polybius be proven a very problematic source that cannot be trusted in its military accounts.

What I am saying is that you either treat him as a source who knows what he is describing and then stick to rank and file or you assert that he is mistaken and go for this proposal. Unfortunately, I do not see anything in between without overstretching meanings and questioning why he miserably failed to give a correct description, that would be understood by his readership.