Well, I have to say one thing as someone who studies the Persian Empire: What mercenaries?
Mercenaries are a weird thing to define, but generally I think everyone understands it as something like "people who join a military for profit outside of official military recruitment." So far as we know, Xerxes didn't hire any mercenaries. If he did, they almost certainly were not Greek because he didn't need to go outside official channels to get Greeks in his army.
Of course, Xerxes did have Greeks in his army. They were a huge component of the Persian force, so much so that some historians interpret Herodotus' account of the Battle of Plataea as an almost entirely Greek affair. Xerxes just didn't need to hire them because they were already Persian subjects.
"Greeks" were not limited to the modern geographic borders of "Greece" in antiquity. Beginning in the Late Bronze Age, early Greeks started spreading out to the coasts of Anatolia, both along the Mediterranean and the Black Sea. In the following centuries, Greek colonies spread along all sides of the Mediterranean from those first cities in Anatolia, all the way Emporion in modern Spain. From the 7th-6th Centuries BCE, most of the Anatolian Greek cities fell under the control of the Kingdom of Lydia.
Cyrus the Great, founding king of the Persian Empire, conquered Lydia in the late 540s BCE. Most of the Greek cities along the western coast tried to breakaway after Lydia fell. A series of Persian generals proceeded to reconquer the region through a combination of sieges, battles, deportations, and accepting surrenders. In 525 BCE, Cyrus' son Cambyses invaded Egypt, in preparation for this, he expanded Persian control to the largely Greek islands of the eastern Mediterranean. In the 510s, the empire was still expanding under Darius the Great. That wave of expansion included the Greek kingdom of Cyrenaic in Libya, the Greek cities of the Thracian coast in Europe, and the arguably-Greek kingdom of Macedon. From 499-492, the Anatolian Greeks went into revolt again and pulled Athens and Eritrea into their allance briefly. This is called the Ionian Revolt and ultimately led to the campaign that ended with the Battle of Marathon in 490 after Persian armies defeated the rebels.
That was the status quo when Xerxes marched west in 480 BCE. Just as he called up troops from everywhere between India and Egypt, he called up troops from these Greek subjects. He particularly relied on the Anatolian Greeks to provide ships for the navy. As he advanced on Greece, he sent messengers ahead inviting Greek cities to surrender and be peacefully absorbed into the Persian Empire. Many cities did so. Some in Thessaly (northern Greece) were even actively inviting the Persians to annex them before Xerxes arrived. Athens and Sparta were specifically exempted from this offer, and the areas where they were most influential formed the alliance against Xerxes, mostly in southern Greece. To their north, most of eastern Greece sided with Xerxes peacefully. The most powerful Greek city to do so was Thebes.
During the course of the invasion, there were a few individual examples of Greek soldiers or Greek ships from the Persian side deserting and going over to the Greek allies' camp, but they largely remained loyal to Xerxes during the campaign. After the battle of Salamis, there were apparently changes in perspective, at least for the Greeks in the Persian navy. Rebels from Samos, Lesbos, Miletus, and other Greek cities/islands in Persian hands started calling on the Greek allies' fleet to support their rebellions that ultimately led to the Battle of Mycale, a series of minor uprisings in Anatolia, and the Athenan-led Delian League liberating/annexing Persia's island territories other than Cyprus.
On land, many Greeks' whose cities sided with Persia served them for the first time on the battlefield at the decisive Battle of Plataea, where they were ultimately boxed into the Persian camp and slaughtered. Following Plataea, the Greek army moved north to Thebes and besieged the city, offering to spare the city if they handed over the pro-Persian oligarchs. Eventually, Thebes capitulated to the demand and the oligarchs in question were sent to Corinth and executed.
Beyond that, no city was punished specifically for supporting Xerxes invasion, even the cities of Thessaly that invited the Persians in. In 478 BCE, the Spartan-led alliance led one more naval campaign in the Aegean to raid Persian coastline and support Greek rebellions, but the land army never mobilized again. After that year, the alliance collapsed when Sparta pulled out (taking the Peloponnesian cities with them). The Delian League emerged from Athens and their allies that carried on the fight with Persia, but primarily in the Aegean and Persian territory rather than reprisals against Greek cities. There's some speculation based on a couple lines from Thucydides that Athens contemplated war with Macedon in this time, but it never happened. In fact, Macedon exploited the power vacuum to expand its territory further west after the Persians pulled out of Europe.
Individuals were punished on a case by case basis in the years after Xerxes' invasion for the newly invented crime of "Medizing" (ie accepting Persian support/supporting Persia) but nothing about it was consistent and eventually basically every major city in Greece had Medized in some way at some point.