Pushtighban: Shields will not appear on horseback, will appear on foot. Could let them use it in melee on horseback but I dunno. It'll just be the business of setting up the animation so it's easy to change.
Late Era Iranian Cataphracts - early era resemble the common Parthian/3rd century art of the two handed kontos bearer. Later era upgrade will turn them into the Khusrow I relief style cataphracts with shorter lances, swords, bow and shield. Your Pushtighban and Gyan Avspar will remain two handed kontos style guys, rest assured. I need to also remove the plain unadorned shields from these guys
Early Era Iranian Cataphracts: Will not have shield on the back on horseback, will wield shields when dismounted. Roughly 50% unarmored horses, 50% rawhide lamellar or felt or quilted armor.
Gyan Avspar: Same deal - shield not on back, only appears when on foot.
Zhayedan Immortal Cav (renamed to the title David Nicolle gives them in his book, and I find his research a bit more trustworthy. But I forget the Pahlavi term he used atm):
Azadan Noble Horse Archers (Elite Savaran replacement - no more silly crossbow cavalry): Same deal as above, shield doesn't appear on back ect.
Dehghans (Minor gentry, T2 horse archers) - May keep shield on back as they do use it from horseback in melee.
Nomadic Nobles/Notables: Replaces your T1 cataphracts (T2 are Early/Late Era, T3 your Pushtighbans, T4 your Gyan Avspars). Represent less settled or non Pahlavi/Sassanian horsemen. If there's a better term that could denote vassals at large like the Kushans and Albanians and others who are not nomadic (kurdan = nomadic in Pahlavi, no relation to the Kurds as we know them) then I'll change it to that.
Paighan-i Kofan or Hillmen Spears. Folks from the Elburz, Caucasus, Zagros and so forth. Fight more like thureophoroi than sparabara.
Paighan-i Sassani or Sassanian Royal(-ish) Spearmen (Replace the top tier Immortals):
Paighan-i Gund or Army Spearmen. Bread and butter soldiery.
Paighan-i Adurbadagan. Adurbadagan can roughly be thought of as the Caucasus - it originally denoted the specific province of present day Iranian-Azerbaijan (Media Atropatene) but would come to denote the kost-i Adurbadagan, the military district of the later Sassanians which encompassed Media Atropatene, Albania, Armenia, and some of north-western Iran up to Tehran. Modeled after the Galerian Arch persians with the addition of armor. Fight more like Thureophoroi. Also available in swordsmen form.
Paighan-i Raman or Commoner/levy spearmen.
Padishkhwagar Axemen. Padishkhwargar being the Caspian province/region of the Daylami, Gilani, Mazandarani.
Bandagan-i Azadan or retainers/followers/'horse boys' as Marcellinus calls them of the nobles. Your T1 trashy melee unit.