Primus Pilus

[Quintius Clavus]

Kate and Andy make the most sense. I, too, find it much better to treat the Roman Army and it's rank structure as unique to it. In the past, long past, I engaged in a discussion with a friend, who was trying to compare ranks in the armies of the American Civil War with Roman military ranks of the Principate. There were some loose comparisons, but only very loose.
The rank of "centurio" has to be modified by the relative position in the century, the cohort and the legion. For an example, my rank as optio in my legion in Latin (Thanks again for Jasper's help with the Latin!): Optio Secundae Pili Prioris Legionis III Cyrenaicae. I am the Optio in the first century of the second cohort in Legio III Cyrenaica.
Gregory Daly in "Cannae" spends a chapter discussion the each side's military organization and command structure. He cites Polybius, Livy, etc wherever there is a clear reference (such as in describing the role of the "optio"). Daly does state that the "centurio prior" - the first centurion of the mid-Republican legion - had a place at the army's war council. This is the officer who became known as the Primus Pilus of Caesar's time and in the Principate.
Regarding "Pilus": I happened to look up the word via an online Latin Dictionary and their definition was "Single Hair". Which then led me to the thought that the word may have like our modern word "file" come to be used for more than one thing, yet carries a visual similarity. Think about it, a single hair, a single file of men(standing in a single line behind the leader), the tools we call files, etc. Items which are long and thin/narrow. It further leads to a thought/theory -mine - that "pilum" may be a name derived from "pilus" by the Romans (or the Samnites, or whomever else might have come up with the name) for the tall, thin, hair-like javelin. It's just a thought,...
Interestingly enough, I sometimes go at the rank structure of the Roman army (those centurion and below) by discussing pay rates. This is the 1 and 1/2 times, 2 times, 5 times, etc. the base pay of an ordinary ranker soldier. Again, we don't have the full picture, but, it provides a simple heirarchical model. It is a comparison modern people in or out of the military can understand.

Quinton Johansen
Marcus Quintius Clavus, Optio Secundae Pili Prioris Legionis III Cyrenaicae