I have a couple of packs of the Antonine Miniatures skirmish castings that I originally bought for the Parthian army but didn’t get round to painting. I’m now thinking of using them for a skirmish unit to protect my Imperial Roman Legions, so have been doing a bit of reading around the subject. It seems that an auxiliary unit of Cretan archers took part in Trajan’s Parthian campaigns, so there’s clearly an historical justification for a distinct skirmish formation, as opposed to just another big standard auxiliary base . There’s an interesting article here explaining how they would have been deployed:

https://www.karwansaraypublishers.com/en-gb/blogs/gary-brueggemans-the-roman-army-ancient-warfare/the-imperial-armys-skirmish-position