my bet is that they start as a vassal of the seleucids, as does batria, they both declared war on the seleucids at the same time, when they ( the seleucids )were at war in the west of the empire...
By the time they reached the east (partia and bactria, they fought some battles, but soon decided the recognise them as independent kingdoms...), so the partians took advantage of the situation of the seleucids.... i bet it will work out in the same way in Rome 2 :-)