Persian Heavy Infantry

Linear A Cunaxa set represents earlier Persians/Medes from Persepolis so is indeed perfectly suited for Xerxes but sadly not for Cunaxa, also much of the dress is court style rather then battlefield but that wouldn't bother me so much.
 Otherwise any troops dressed in median dress consisting of a short tunic with long sleeves, pants with a close or loose fit would work and armed with a variety of weapons mostly bows, javalins, short spears, and equipped with scale armour of painted rawhide or metal which might be worn under or over the tunic, with typical Persian figure 8 shields etc, this applies to many figures currently available so check out PSR:

www.plasticsoldierreview.com/PeriodList.aspx?period=2#Persia

It should be reasonably possible to produce the early Persians units needed particularly from the Caesar set with little wastage though these are lacking in bow cases but they can added, and the shield to hand connection would need modifying for a better look, but this is a bonus as it allows you to add the correct shields* not be stuck with Hoplons as in the later Hät set though a few may not be out of place. (*This is a good reason to have seperate shields in a set as it allows you to vary the era by simply having a different shield or none at all)
Add to these suitable light Infantry and Cavalry from Hät and Zvezda and I think you may have an Army worthy of Mordor.....

Incidently all the good sets work well together Caesar, Hät and Zveda persians all being a similar size and bulk, cant comment on linear A though I would buy a set or more if the fit with the others was good despite the huge spears as I would change these.