To offer a bit of an alternative to u/XenophonTheAthenian's rundown on the situation in Rome, there does seem to have been some ethnic clustering in Classical Athens.

In Athens, resident foreigners (called metoikoi or metics, literally "those who live with us") were inferior to citizens in many ways. They were not allowed to take part in political life; they were not allowed to marry citizens or buy land; they were subject to a special foreigner tax. More relevant to your question is the fact that they were not allowed to buy plots in the city to build houses. This meant that they tended to cluster together where places could be rented on the cheap. Given that the laws prevented them from becoming farmers, most of them worked as skilled labourers, artisans, or sailors; inevitably, most of them lived in the harbour, Peiraieus, rather than in Athens itself.

It is in Peiraieus, too, that we find a sanctuary dedicated to the Thracian goddess Bendis. It seems likely that part of the area around the Mounychia hill near the harbour was something of a "Thrace-town", settled by sailors, workmen and mercenaries from the northern fringe of the Athenian Empire.

In addition to this, early on in the Peloponnesian War, Athens took in a significant number of refugees from its ally Plataia, which was besieged by Sparta. These people effectively formed their own community within the city, and at least one source suggests that they had their own community leaders and hung out on their own corner of the agora (the town square and centre of public life). Presumably, the Plataians would have clustered together in a particular neighbourhood when they arrived at Athens - possibly in one of the shantytowns that cropped up between the Long Walls to provide refuge from those who fled the countryside ahead of the yearly Spartan invasions.

I think this earlier question should be of some use to you