Roman use of mercenaries

[L C Cinna]

The problem with your original question is that during Caesar's time in the late Republic the leader of an army was responsible for the army so the organisation of support troops was completely up to a man like Caesar or Crassus. So probably most of their "auxilia" were mercenaries or soldiers provided by tribes with which the commander made a more or less personal contract and not necessarily a contract between the state and the tribe.