At the top of this section of the forums is a Map Thread, the very first map shows the provinces where your reformed places can be built. As the parthians, I would say go with the most civilized options you have. Cilent State, if not then pastoralism. They only need nomadism in the few places where it gives them good native steppe units. Unlike the Saka/Sarmatians who need it for their best, the Parthians get their good units from MICs.