Pausanias on Achaean armament, ca. 200 BCE

[Paullus Scipio]

For the Battle of Sellasia, see a forthcoming account in Ancient Warfare magazine, which will pay particular attention to the topography, and how this produced some unorthodox troop formations.....
In pitched battle at least, I do not believe there were any fundamental differences between Peltasts and Phalanx, save perhaps in richness of arms. That those same troops could, if the mission/task called for it, discard their body armour and sarissa, and carry out a 'lighter' mission, I don't doubt either - we are given plenty of examples, as Inaki says, of them carrying out these types of operatiom, and such elite troops had been doing such things since Alexander's time.
The real question is to what extent did these dual function skills extend to the Phalanx as a whole?
Clearly Alexander's full-time professional pezetaroi, with all their experience , were able to do this, and elite units...possibly regular units too ( such as the Chalkaspides /Bronzeshields), but I doubt if the Macedonian Kings went to the trouble and expense of training the 'citizen militia' to this extent, if only because money was always short, and you didn't need a whole army to carry out these specialist tasks.....a factor recognised by modern armies too...................