You might try these two sites:

The Roman Army of the Late Republic ; this has a vaaaaast list of Roman military terms;

Caesar's Commentaries translated (or mistranslated).


Incidentally, the term "Old Sweat" might not be correct in this context; Julius Caesar was indeed an experienced soldier, but his first military appointment was as a Legionary Tribune, which might equate to a Major in present-day armies; the system by which men of suitably fortunate birth in the Roman Republic were appointed to military and civil posts was both very rigid and very complicated.

An "Old Sweat", in British Army usage, is usually a long-serving ranker or Junior NCO, with long experience of soldiering but no aptitude for promotion. i.e. the opposite of the young Julius Caesar. (I never heard the term used in anger, and rarely see it in literature except in the context of World War One, or peacetime service between the two World Wars).