Only units you can build yourself can be bribed to your side. I've found this out playing the Seleucids: I could bribe from a Parthian stack of horse archers, eastern infantry and one kataphraktoi only the katatanks (who eventually can be recruited). When I bribed a Greek stack, I gained a unit of Militia Hoplites, while the regular hoplites disbanded. That combined with the fact that influence is no matter neither is culture (Parthians have a different culture) in bribing, Im quite sure this is right.