My usual Parthian setup goes like:
4 Horse Archers -Ha
4 Heavy Cav -Hc
4 Slingers/javelins or 2 slingers, 2 javelins -S and J
4 Sword units or hillmen - I
2 Spears -Sp
2 Ballista - B
JJ
Ha Ha SpIIIISp Ha Ha
Hc Hc BSSB Hc Hc
Probably not the best but it works for me.
As Parthia, I go with 15 HAs + 4 melee/heavy cav + the general for most field armies. Then, I have one dedicated city grabber army filled mostly with Parthian swordsmen + ballista + archers.