Have you checked AD&D Glory of Rome out? (I haven't).
Roman Army rank structure was pretty simple. Under the Republic, the two
consuls would alternate command. Assisting them were legates, quaestors and
tribunes of the soldiers (also annually-elected positions). All of the these
could command Roman soldiers, but there were not in the legion's chain of
command. Once the Empire came around, these positions were either elected or
appointed. The army would consist of legions (disciplined heavy infantry
standing formations) and auxiliaries hired from the local area. The general
structure was to have centuries of 60-100 men, commanded by a centurion. 6
centuries made up a cohort, and ten cohorts a legion. The most senior
centurion in each cohort also led the cohort, and the senior cohort leader was
also the acknowledged leader of the legion (primus pilus, or "first spear").
There were no commissioned or non-commissioned officers, the centurions
performed all leadership roles, and advanced by seniority through their
legion. Non-legionaries (light infantry, slingers/archers, cavalry) would
also be organized in centuries and cohorts, but not legions. Usually, a
legion had some cavalry and a variable number of siege artillery pieces
attached permanently.
Under the Republic, there were only 4 legions, and they were created and
dissolved as needed, all of the soldiers were landowners who could afford to
bring their own armor and weapons. After Gaius Marius (ca.100 BC) legions
would be recruited by individual nobles/consuls (Gaius Marius, Sulla, Pompey,
Julius Caesar, etc.) for specific campaigns from the poor and landless and
equipped at State expese, but by the end of the Republic, they were
maintaining them past the end of the emergency. This allowed a major
expansion in military force, more land under permanent Roman control (Spain,
Gaul, N. Africa), and major portions of the army who felt beholden to
individuals, and not the Republic, and so the Republic collapsed into Empire.
Basic legion/auxiliary structure did not change much, and only 20-25
legions were maintained throught the "stable" period of the Empire. Major
changes came about once the Empire split into East and West. I'm no scholar
of that time period, but I can point you to a really great (IMO) series of
novels covering the late Republic: "Masters of Rome" series by Colleen
McCullough, covers politics and personal relationships from 108BC. The
glossaries in the back are massive and very useful. I want to a Roman game
someday after reading these.

Lee H.