"Roman skirmishers during Early Imperial period" Topic

Do we have much info about how skirmishers were used by the Roman Army during the early Imperial period? I also specifically want to know which ethnic groups were favoured by the Romans when it came to recruiting for skirmishers and light infantry.

Skirmishers were always useful for scouting, fighting enemy skirmishers, and annoying the enemy battle line. However, in the early imperial period most skirmishers in a Roman army were recruited from peoples subjugated by the Romans or allied to them. Thus, Roman armies could feature slingers from the Balearic islands, Cretan archers, Syrian heavy archers, Spanish or Numidian light cavalry, Celtic slingers or javelin men, and many others. Later nomadic horse archers were used--particularly in eastern armies.

Were they more recruited as numeri?

I know Cretean and Syrian archers had their own cohorts as did Spanish and Numidian cavalry. I don't recall any units of slingers, though they do appear in Trajan's Column.

I think they were often numeri.

I think it is interesting how much we don't know about the Roman Legions. I think it was a mixture of local commanders having a lot of authority to do things their own way mixed with the fact that some things were so commonplace to them that no one really bothered to write it down.

From what I have read by the early imperial period auxilia tended to be quite organized and often operated in standardized units. But the Romans often used ad hoc units of "socii" or local allies or "Foederati" which were mercenaries. I think they would have been suited for skirmishers because they would have been familiar with the local terrain.

During the early Imperial period I think the Romans had an almost obsessive fixation on field battles with Heavy infantry. Anything else felt very "un-Roman" so I would not be surprised if that work was not farmed out to local allies and mercenaries. Records during the Parthian wars often talk about allied Armenians providing the role of light and medium infantry because the legions were not well suited for that kind of fighting.

One personal suspicion that I have is that I have read in several places that legionnaires were often required to train with bows and slings. But don't every read about them using them (maybe hunting?). I would not be surprised if occasions a few centuries from a reserve cohort weren't broken up into a skirmish line when no suitable auxilia or local allies were available. But that is just a guess on my part.
As far as what ethnicities, I think the Roman saw the value in light infantry but didn't give it much prestige. So I don't know if they would have required much for it outside of its slinger and archer auxilia (and I don't know if they fought as skirmisher). I suspect the ethnicities of skirmishers would be local to where the legion was stationed.