It depends on what exactly you want to do :
-If you're going to fight a absolutely historical Thermopylae ,Then I think EB1 is not good choice for that ,As it begins at 272BC not 490BC !
Hegemonia:TW could be better choice if it'd been released !
Rise Of Persia is the closer mod to that era (Although I finishes before the time of
Xerxes rise to power) and It is in Beta2 ...
-If you want a historical battle by whatever EB1 can provides you ,Then here is what I suggest as army composition of both sides :
1
Eastern mercenary general ; Although there are some differences between this one and a real Persian bodyguard unit at Thermopylae.
2
Asabârân-î Mâdâën ; As for Cavalry contingents of Immortal corps. However ,They should have Javelins !
4
Asabârân-î Haurâvatîsh ; These are closer to persian cavalry at the time of Thermopylae.
8
Nîzagân-î Êrânshahr ; Although they have lack of armor ,Experience and also Shield but still are the only choice in EB1 for Immortals !
4
Native Spearmen ; for Allied forces
1
Levy Hoplite ; For Greek allies
And as for Hellen side ,3 Spartan ,3 Greek hoplite and one greek general
As I said ,EB1 is not really good choice for hiistorical Thermopylae ,Especially Persian side is not close enough to reality. EB is good (Better to say excellent

) for depicting battles in the mod time span.
-But if you want a fantacy Thermopylae like a one million persian army and those things ... Then grab 1-3 Sprtan hoplite and put them infront of a full stack of Native Spearmen and Perian archer-spearman and watch them how they're being teared apart
Cheers
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