About slave.
Slave in nomadic army it is unlikely.
Slavery begins to rise in urbanization (primary for hard work and sexual entertainment). An army of slaves was formering when large military campaigns (for example Huns).
Ancient steppe nomads don`t used slaves (used but sporadic and unknown case).
Slave was (not for arrmy, not for work) - but like for sale and Exchange (sale to neighboring folks and tribes). Or for execution (political, personal animosity etc)
About saka army bonus in mod.
The main things for saka it is arch and horse.
Maybe need to make big radius shooting of archers? or (and) shooting accuracy? or (and) horse rejection (maneuver) from the enemy?
And horse speed (nomadic women horse archer)?
What do you think about bonus?
Is it posible?