I wish to underline that most sagitarii were of asian (middle-east) or indo-iranian (sarmatian) and i dunno if in the 2 dacian cohorts they had archers, but what i do know that dacians were some of the best archers, axe-throwers and swordsmen (look below why). So although cretan archers may be some of the first archers to be encountered, they were not the best or most common... especially in later periods the middle-eastern archers and horse-archers and chariot archers were most common, and these were all from Egyptian and Persian areas...
Anyway, now back to my beloved people... in dacic tradition there was a festival in which a noble or royalty got married... and all maidens were "initiated" by their parents who were "tested"... Dacian girls grew long hair in a ponytail. They stood back to a tree, and their warrior father had to shoot his bow to hold the ponytail to the tree, and then they had to throw a hand-axe to chop the ponytail off... You can imagine what would happen if he hit the girl... dishonor and sorrow to death and beyond. If they missed the hair the girl could not marry... So you can logically deduct that they HAD TO BE the best in archery and axe-throwing if they had daughters
However considering they most often then not employed in guerrilla warfare and covert operations, archery, stealth, axe-throwing, and sword fighting was a huge must... and for this exact reason they used light armor (the heaviest armor was something similar to lorica squamata, with medium armor being lorica hamata).
Now the thing is that there's little info about the dacian cohorts so i don't know how many actual archer units there were, but give any dacian a bow and he'll shoot at least as good as a hun...
Forgot one thing... yes, archers were auxialiary units, as few romans (if at all) were archers and all foreign units were auxiliary.