Patrick, I can say that Pallava and Cola rulers probably used Tribal people as soldiers as testified by some sculptural reliefs of "victorious Durga", where two worshippers at her sides are represented with somatic traits of Tribals---and self-cutting their heads or flesh from their thigh. I think your question is interesting.

Paolo

Paolo E. Rosati
Oriental Archaeologist
PhD candidate in Civilisations of Asia & Africa
Section: South Asia
Dep. Italian Institute of Oriental Studies (ISO)
'Sapienza' University of Rome

paoloe.rosati@uniroma1.it
paoloe.rosati@gmail.com
Mobile: (+39) 3387383472
Skype: paoloe.rosati

Please forgive the possibly naive nature of this question, but was it the case that all warriors were considered to be of the same kṣatriya class? Or, was it possible that the 'officers' were kṣatriyas and the 'foot soldiers' were perhaps of a different caste, i.e. enslaved śudras forced to fight? Is there any discussion of the militiary organisation according to ranks, size  and hierarchy similar to the table below? 

Thanks in advance.