by shooting alone or also by combat/melee? Huns after all have ME or if skirmisher 'Huns' Cantabrian and so skilled.
Largely by shooting I would take from the sources. The Vandals had 2,000 horsemen dispatched on a right hook at Ad Decimum, they encountered the 600 Huns sent to cover Belisarius' left flank and were routed/destroyed. Outnumbered by more than 3 to 1...the tactics likely would have been the usual evade, mounted archery, disrupt and pick-off stragglers and only close to melee if favourable.
Procopius (History of the Wars III (the Vandalic War) xviii, 12-19:
At the same time Gibamundus and his two thousand came to Pedion Halon, which is forty stades distant from Decimum on the left as one goes to Carthage, and is destitute of human habitation or trees or anything else, since the salt in the water permits nothing except salt to be produced there; that place they encountered the Huns and were all destroyed.
Procopius calls them "Huns" or "Massagetae" interchangeably, exact terms are not the issue: this was an expeditionary army so it's likely the best available were chosen. Losses were either very light or non-existent on the Hun side. Later in the campaign the Vandals attempted to bribe the Hunnic contingent to switch sides...indicating that they were seen as a potent force. Procopius when mentioning the Huns (or "Huns" if you prefer) generally has a high opinion of their combat ability. They ("Huns") are referred to often, in the Persian Wars, Vandalic War and later in Italy (Gothic War). Almost certainly different contingents of course, but mentioned in positive terms.
I would absolutely have an option to upgrade to Skilled (Flexibles) as their mounted archery skill is noted in the sources. The Eastern Roman/Byzantine army was increasingly reliant on mounted archery; they were no slouches in the field, the Bucellarii in particular noted for their mounted archery skill.
is the primary purpose of that line to make them skirmishers or make them cantabrian skirmishers? most lists limit skilled or cantabrian focus.
I have no clue on Early Byzantine army composition but your assumption seems to be 'Hunnic' cavalry could be either skirmishers or flexible types whereas my assumption is Byzantine armies preferred Hunnic mercenaries , the flexible types, but on occasion had to settle for other nomads, pseudo-Huns, to stand in for limited availability of actual Huns.
I can't speak to "purpose". Again, terminology or names is not the focus: effectiveness is demonstrated by the sources. My question was simply...
If one unit is allowed to be Skirmisher, why not both? The Skirmishing Huns or Flexible Huns are exactly the same troops, the difference in classification is an artificial wargames-rules-thing, not reality. But it does affect the way the force is built and functions. If the answer to the question is just.... "err, because", then fine: I simply wondered why.
For the record:
(a) I don't have an Early Byzantine army (yet....) I just have an interest in the period.
(b) Cantabrian is absolutely a no-brainer upgrade and is too effective for it's points IMO. I would make it more expensive or change the upgrade to rule to make it "shoot on White, S counts as a wound, no upgrade vs. Superior", etc. etc. Just saying.
CdlT