Spartan Mora regiments at Battle of Plataea

[Paullus Scipio]

It is entirely possible that the perioici were deputed to “homeland defence” – anything is possible given the equivocal nature of the source material. There seems – mostly – to be a regiment or two of the army generally left at home (or at least those above 35) with whom they might have been deployed. Certainly you would not leave the city’s defence in the hands of helots and perioici alone.
...with this I would agree...though I assume 'above 35' is approximate, and varied on occasion.
The unequivocal evidence comes down to times of crisis for Sparta: the Athenian occupation of Pylos and the battle of Plataea. In both instances Sparta is clearly described as mustering the perioici as part of the army to confront the crisis.

At the beginning of the Peloponnesian War Sparta musters an army of invasion at the Isthmus (as with Plataea). Thucydides, at 2.11.1, has Archidamus address his generals with the following words:

‘Peloponnesians and allies, our fathers made many campaigns both within and without Peloponnese, and the elder men among us here are not without experience in war. Yet we have never set out with a larger force than the present; and if our numbers and efficiency are remarkable, so also is the power of the state against which we march.

“We have never marched out with a larger force than the present” is, one thinks, Thucydides’ method of conveying the size of this force. Just how large it was it not readily attainable but, if the above is true, one needs to go back to Plataea to find a comparison. All Thucydides tells us just what the muster was for this force at 2.10.1:

...the quotation seems to be in reference to the time of the 'elder men' and their 'fathers' - i.e. "living memory"

The several states were ready at the time appointed and assembled at the Isthmus; the contingent of each city being two-thirds of its whole force.

This would, indeed, correspond to what we know of the Plataean army. I would suggest that it most likely counted Perioici amongst the Lacedaemonian two thirds.
...it probably doesn't matter, since the result is the same in terms of numbers, but the fact that the Spartans and their Allies formed a single phalanx on this as on other occasions, is no evidence that "allies" served in the Spartans own cherished 'Morai' - any more than British and Australians served in the same 'regiments', although part of the same 'division'/Army in WW2 - or Americans and British served in the same units during the Normandy invasion, though forming one army....

Be that as it may, clearly, Mantinea in 418 was the gravest crisis Sparta had faced since the Persian invasion. Alcibiades, giving note of his abilities, has not only roused Argos to action but has also detached League allies from Sparta and put them into the field. A loss on the field here will have occasioned Sellasia-like results. And, like Sellasia, I rather suspect that the Spartans will have mobilised as many of her resources as possible. To my mind that would include periocic hoplites.

...given Kinadon's (exaggerated) remarks that 'perioikoi' hated 'Spartiates' and would "eat them raw", clearly there was a prospect of treachery in any 'internal' war in the peloponnese, as indeed ultimately happened among Sparta's allies after Leuktra. As I alluded to earlier, there are reasons to think that, as a rule, 'perioikoi' were not co-opted to serve in the peloponnese against peloponnesian enemies prior to Leuktra...indeed, the nature of the relationship may have been that 'perioikoi' were only obliged to serve "in defence of Lakedaemon" against external enemies or something similar.....
The other possibility here is the use of the term “allies”. At the time of the King’s Peace prior to Leuktra Agesilaos insisted on Sparta’s right to sign on behalf of the allies – included in this was the legal nicety of the periocic “cities”. So to Xenophon describing the return of the army after the calamitous defeat at Leuktra an army that (at 6.4.17) included “those who were forty years beyond the minimum military age”:

And proceeding with very great difficulty, since they were withdrawing at night and in fear and by a hard road, they arrived at Aegosthena in the territory of Megara. There they fell in with the army under Archidamus. And after waiting there until all the allies had joined him, Archidamus led back the whole army together as far as Corinth; from there he dismissed the allies and led the citizen troops back home.

Again, given desperate situation indicated by the call up, I’d think it highly likely periocic hoplites were part of Archidamnus’ force if not the original force under Kleombrotus.

It is just possible that the perioici are referred to as “allies” this being the legal nicety Epaminondas so enraged Agesilaos about.

...I would agree that this is possible, and that 'perioikoi' were technically 'allies' of Sparta, and your point is well made that Agesilaus was infuriated that Epaminondas/Thebes insisted that they should sign on behalf of all the Boeotians - thus equating Thebes 'recent' dominance of Boeotia with Sparta's 'ancient' dominance/pre-eminence in Lakedaemon. In addition, Agesilaus acceding to this would have 'legitimised' the Thebans control of the Boeotian cities - when their policy was to 'uphold' the independence of cities such as Plataea and Thespiae......
In which case, 'perioikoi' being 'allies', they would not have been serving in the Spartan 'Morai' ( see above) nor continually being mentioned separately ( as at Sphacteria and post-Leuktra), and were they 'integrated' into the Spartan Army rather than serving alongside as Allies, why did Agesilaus not point out this major distinction - that 'perioikoi' were part of the Spartan Army/state, but Boeotians weren't part of the Theban Army/state ? This failure to state an obvious difference argues strongly that 'peroioikoi' served alongside as allies in the same way as Boeotians served alongside Thebans.....
As to the formulaic "dismissed the allies and led the citizen troops back home", it is attractive to consider that 'perioikoi' were lumped in as 'allies' and not distinguished, but then it becomes necessary to explain this:

Xenophon varies his formulaic "dismissal" phrase and this time he dismisses "the Spartans to their homes and the Perioikoi to their various cities" (XH VI.5.21), whereas before the defection of the Peloponnesian allies, there is no mention of perioikoi in connection with campaigns against peloponnesian enemies. Further support for this idea, although there can be no certainty, is that in a campaign against the Argives in 391, Agesilaos " disbanded the army of his allies and led the citizen army back to Sparta" (XH IV.4.19 ).Another example is after a campaign against Phleious in 379, he "allows the Allies to disperse and led his own troops back to Sparta" (XH V.3.25)........ presumably only Spartan troops would return to Sparta. In both cases against Peloponnesian enemies, no 'perioikoi' are referred to.

Here, for once, the 'perioikoi' ARE referred to, and the explanation may be that on this occasion ( after Leuktra ), the only 'allies' left are 'perioikoi'.......
At all events, all these expressions differentiating citizen troops from allies/perioikoi clearly suggest they served in separate units....