Spartan Mora regiments at Battle of Plataea

[Paullus Scipio]

Paralus wrote:

And whereas I would not dispute Herodotus’ lack of travel to some places in his “History”, I imagine he likely visited Plataea.

...in fact, Herodotus makes it fairly plain he had almost certainly NOT visited Plataea - in speaking of the graves he says "...so far as my information goes..." and "...which I am told.." (H. IX.84 )

Herodotus goes on to list the contingents over to the left wing held by the Athenians. We have 5,000 homoioi alongside of whom, to their left, are the Tegeans. In both instances Herodotus uses the term Spartiate.

I don't think this interpretation, which is the other piece of information on which your "rear ranks" hypothesis rests, can be correct either.We have seen already that Herodotus is quite capable of overloking 'perioikoi' altogether (Thermopylae)....but here he does not, fortunately. In elaborating the Greek line he says:-
( H.IX.29) "The order was as follows: on the right wing were 10,000 Lakedaemonians, of whom the 5,000 from Sparta were attended by 35,000 light-armed helots - seven to a man (there are a couple of things to note here. First, all authorities doubt the '35,000' figure for the Helots....it may be that each 'homioi' was allocated 7 helots to his 'Kleros', and H. has applied this to guess the number of helots present, but they would not all have followed their master to the battlefield. Second, the 'Spartan' army need not have been all 'Homioi', for it may very well have included 'hypomeiones' as well as 'Homioi' despite H.'s assumption.....and the secretive Spartiates were not going to disillusion H. or anyone else that there were fewer than 8,000 'Homioi' ( if there were ! ). Next to themselves the Spartans ( spartiatoi/spartiates) stationed the Tegeans - 1,500 'Hoplites'- as a tribute of respect to their worth...

Naturally it is the 'spartiates' who do the ordering/choosing, nor should it be taken any more literally than if he had said 'next to the spartans[read spartan army], Pausanias stationed the Tegeans'...the natural meaning is that the Spartiates/Homioi stationed the Tegeans next to the 10,000 strong Spartan/Lakedaemonian army, and I really don't think one can infer 'perioikoi' in rear ranks from this passage.
As to 'perioikoi' joining in the ranks of the Spartan 'Morai', why would this be done, when there were obviously plenty of non-homioi Spartans? Surely one would expect 'spartan' manpower to be drawn on before 'strangers' ? Then there were practical matters. The Spartan advantage was that they were 'professional' soldiers who were constantly drilled to perfection by practice -practice-practice. How could a 'perioikoi' 50 miles away join in this ? ( and rest assured, anyone familiar with drill will tell you that you can't mingle 'trained' and 'untrained' or chaos is the result.Nor can you train 'separately' and then come together) In addition, 'periokoi', or at least their upper classes seem to have had estates/kleroi allocated to them at some time, but not Helots, therefore they needed to work/manage their estates/farms themselves and couldn't train in Sparta anyway. Non-Homioi 'Spartans could readily be required to drill daily ( perhaps after work...c.f. English yeomen being required to practice archery).That this professional drill/teamwork was their forte, we are told again and again by Herodotus, most notably when the exiled Spartan King Demaratos tells Xerxes: "..fighting singly, they are no worse than other men, but fighting together they are the best soldiers in the world." (VII.104.4). Were the ranks filled up with non-professional 'perioikoi', and the 'Homioi' no better fighters than other men, where would the Spartan advantage lay ? ...Further, as the Thebans discovered, and Paul B. has referred to, simply putting a 'hard edge' on the front of the phalanx simply did not work....