Herodotus' description of the Battle of Plataea gives a fair deal of insight into the reasoning driving the tactical practices of the Greeks at the end of the Archaic period. First and foremost, it would appear that defending the flanks is not a prime mover in the choice of battlefield:
"When they came (as it is said) to Erythrae in Boeotia, they learned that the barbarians were encamped by the Asopus. Taking note of that, they arrayed themselves opposite the enemy on the lower hills of Cithaeron. " (Herodotus 9.19,
http://www.perseus.tufts.edu/hopper/tex ... apter%3D19 )
"Presently they resolved that they would march down to Plataea, for they saw that the ground there was generally more suited for encampment than that at Erythrae, and chiefly because it was better watered. It was to this place and to the Gargaphian spring which was there, that they resolved to go and pitch camp in their several battalions; [3] They took up their arms and marched along the lower slopes of Cithaeron past Hysiae to the lands of Plataea, and when they arrived, they arrayed themselves nation by nation near the Gargaphian spring and the precinct of the hero Androcrates, among low hills and in a level country." (Herodotus Histories 9.25,
http://www.perseus.tufts.edu/hopper/tex ... ht=plataea )
When they plan their move to the Island, it is to both to have access to a water supply and keep the cavalry off of them:
"To that place then they planned to go so that they might have plenty of water for their use and not be harmed by the horsemen, as now when they were face to face with them" (Herodotus 9.51,
http://www.perseus.tufts.edu/hopper/tex ... apter%3D51 )
However, they are not able to make the move immediately, and suffer:
"Having made this plan, all that day they suffered constant hardship from the cavalry which continually pressed upon them. When the day ended, however, and the horsemen stopped their onslaught."
I cannot help but think the horse pressing upon them must have included attacks on the flanks, although this is not explicitly stated. That said, they must have been able to arrange to protect themselves somehow.
Now Herodotus does clearly state that the Persians fighting the Spartans were not co-ordinated in their attacks:
"Now the Persians were neither less valorous nor weaker, but they had no armor; moreover, since they were unskilled and no match for their adversaries in craft, they would rush out singly and in tens or in groups great or small, hurling themselves on the Spartans and so perishing." (Herodotus 9.62,
http://www.perseus.tufts.edu/hopper/tex ... apter%3D62 )
Its also worth noting that the there were 30,000 light troops with the Spartans to help protect them against trouble as well...
Its also my belief that small groups of flankers are largely ineffective against a large formed body, either mobile or not. I've been on both sides of such exchanges in various recreation and re-enactment battles, and have found that the bigger body shrugs off such attacks fairly readily. This is particularly true if they are moving. I once participlated in a poorly co-ordinated attack by 200 men on the flank of a moving formation of close to a thousand. It was like being cheese hit by a flying grater, and by the time the survivors had regrouped the moment was past.
Have fun!
Cole