For the many discussions about what exactly the 'Kardakes' were please read
I have fielded these units as Kardakes with hoplite panoply or in other words Persian hoplites formed to try to counter Alexander's pikes and to supplement the mercenary hoplites. Darius' only reliable infantry were the mercenary hoplites and the Persians needed as many as they could get hold of. They had watched Philip II sweep through Greece and they, the Persians, knew that they would be next. I would suggest that training in close order was rushed through, perhaps under the supervision of Memnon or Mentor, to prepare the Persian army for the expected assault.
These are based to Impetus and are just three of the six I plan to do to allow them to be doubled up.
The army being planned here is primarily aimed at fighting the Macedonian advance guard army of Parmenion sent into Persian territory in 336 BC.