It may well be deliberate.. but the infantry minima for the early Iranians, combined with only a single sub general, make it difficult / expensive to field the mixed infantry units.
Minima of 12 of each for spearman and archers means that to get mixed units toy need to take at least 12 archers in the main list to go with the minimum 6 spearmen. As there is also a minimum of 1 tug of cavalry.. the internal ally is also very limited.
As there is only a single sub general, the minimum number of archers is actually 24 if you have more than two generals.
Would it be better to have the mixed units as a separate line with a separate minima ratio
For information, in the 2020 lists, the number of sub-generals has been increased to 2.
As the mixing is not an all or none situation I am not sure you are correct. You could take 12 archers in mixed units with 6 spearmen and then take 6 spearmen separately.
Richard
But is a mix of mixed and un mixed units historical?