salve frater

that sounds very interesting!
what was the general region called at that time? obviously Pannonia, but any (perhaps more widely known/Roman contemporary, eg. Dacia?) other names that might help? i have a set of army lists from a historical research group that might have useful info on auxiliaries and things, if i know which army to look for, as any garrison troops in the area in the late 4th century would probably have been frontier auxiliaries or limitani, something similar.
aha, just had a look at Wikipedia and Illyricum and Dacia would be the right area, i think. if you haven't checked this page (http://en.wikipedia.org/wiki/Pannonia) it's probably useful. good choice, the period shortly before it was ceded to the Huns, a time of change!

can't specifically help with any other bits i don't think; slaves could be and were enlisted by the Romans during the 2nd century BC (2nd Punic War) but not again so far as i know, service for citizens was of course voluntary. try looking for the Osprey series of books on the military side though, they're usually the best. i'll get back to you about the army lists, which have general stuff on the Romans too...

ave atque vale