There were greek mercenaries in the Persian army, not to mention the other Greek populations the Persians were overlords of. Not all Greeks were against the Persians. Also, after the war, themistokles was later ostracized and served the Persians for the rest of his life. https://en.m.wikipedia.org/wiki/Themistocles

Go to the sub section titled "Greeks in exile to the achamaned empire" I'm sure you could find more info for other Greeks serving the Persians before the war following that source