Now when it comes to the parthians...
You need armies of horse archers and heavy cavalry, end of story. The Armenians you can fight with a standard Army (although a bit more calvalry never hurts), but the Parthians are just a whole different ball game. The babylonian horse archers work very well, but as I said, you need to back them up with heavy cavalry, be they cataphracts or mercenaries. In my
campaign I lost two full stacks to the Parthians, along with plenty of other casualties, seeing as I was at war on 5 fronts, which will happen to you soon with the Pergamese and Armenians. The thing is, you have to fight the Parthians with cavalry, foot archers and phalanxes are useless, get Machaimorai Epilektoi to engage their infantry, which are like peasants with pitchforks anyway...
Gook Luck mate, and +rep for being a seleucid supporter!