Auxiliary Infantry Units and Rank

The Roman legion was primarily a heavy infantry force. The roles of missile troops, cavalry and light infantry were filled by the non-Roman tribes more suited for those tasks. During Caesar's time non-citizen auxilia were not a regular part of the army. Germans, Gauls, and other warlike tribes were called upon when they were needed. In Augustus' reign the auxiliary units were incorporated into the army on a permanent basis. The main inducement for a non-Roman to enlist was the prospect of obtaining citizenship for him and his family. Citizenship was valued because it greatly improved a man's social standing and gave him the full protection of Roman law. An auxiliary soldier would serve 25 years before being honorably discharged and granted citizenship. Occasionally, the heroism of an entire unit would convince the emperor to award them citizenship before the end of their term. Auxiliary equipment was varied, but was generally of a similar quality to that of the legions. It seems, the legions often looked down on auxiliaries as inferiors, though they played an indispensable role in manning the empire's frontiers.

Auxilia Infantry Units
Cohors consisted of 500 men.
Cohors Milliaria was 1,000 men.
Numerus was probably 300.

Auxilia Infantry Ranks
The commanders were most commonly Romans, though tribal leaders also could hold these positions.

Tribunus Cohortis
The overall commander of the non-citizen auxilia infantry.

Centuriones
Roman citizen centurions were put in command of the non-roman troops.

Princepales
Non commisioned officers appointed to aid the officers.

Immunes
Soldiers with extra responsabilities, and had immunity from the less desireable duties.

Milites

Common foot soldiers.

References:
1. Greece and Rome at War, by Peter Connolly
2. Auxilia of the Roman Imperial Army, by G. L. Cheesman
3. The Complete Roman Army, by Adrian Goldsworthy