Totally not a buzz killington, the challenge is satisfying, gives me more to think about.
Shame Gelonus may be fictional, do you think that people would have believed Herodotus? It could lead to catastrophe for anyone who tried to reach it if it didn't actually exist!
Regarding the cities, if the settlements in Dobrogea (Admittedly not as exposed, but still) are anything to go by, then there was a large amount of local trading, which is what the settlements were based on, not just inter-urban trade, in fact that seems to be the minority (at least there), even if they exported grain to Athens.
Also, those north coast greek colonies stood tenaciously, against all comers (including the Macedonians!... Just) I wouldn't be surprised if that tenacity and resilience would also be a characteristic of settlements on the Caspian as they could trade with much closer partners, but regardless of that trades importance, I would be surprised if they fell, especially if the area was purposely urbanised. (Though I guess that could become the reason, I just can't see how right now).
I do find the hun aspect interesting, partially because I can see a successful greek/urbanised society in the south-russias (it seems to be growing now) getting information from the Chinese, that information could be vital, and perhaps make steppe tribes much less dangerous, as they are less unexpected. - But that would require far more work to give any credence to.
to bring @DominusNovus back in,
Both of you are entirely right though, most likely a greek-dominant society would have difficulty without SERIOUS backing, and of the right type. I personally like the Macedonians/Thracians because of their cavalry traditions, and their relation to the steppe tribes. That and (whilst I don't assume common talents), Alexander did manage to pin them down and show them who was boss. AND AFAIK, the Macedonias, whilst culturally conservative, were willing to adapt to new military tech, and had their own cavalry, so they might be willing to see, then adopt that style of warfare in the area. (Might).
An alternative to more colonies existing and being independent states, or a series of symbiotic states is that it lead to an early-moskva/mongols situation, where the city states manage to assert themselves after a long time subservient to another.
But in summary
1) This'll be hard
2) They won't find independence easy without a stroke of military or diplomatic brilliance
3) If it works, it'll work well... assuming they don't just start killing each other. Bloody Greeks.
But seriously thanks

It has given me a huge amount to think about, rather than just imagining triremes on the Don/Volga/Dneiper rivers
