The characteristic of fascism are not so much a definition as a cluster of traits. My look so far at Ancient Greece and various oligarchies and dictatorships is that these are neither isolated traits nor a unified cluster. Rather, they have several sub-clusters. Charismatic leaders, mass mobilization, and an emphasis on an aesthetic structure are traits of populism. Belief in primacy of one's group, fear of its decline, the need for integration of a purer community, and belief in the right of one's group to dominate are traits of oligarchy. (Fascism applied them on a national level). A sense of overwhelming crisis and a movement simultaneously anti-radical, anti-liberal, and anti-conservative are traits of a movement driven by fear of rebellion from below and the need to forestall it. Exaltation of youth, mobilization, and a favorable attitude toward violence are traits of a paramilitary party seizing power.
So we have seen in Peisistratus many of the populist traits of fascism (and some of the paramilitary ones as well). In Sparta we have seen many of the fear-based traits. And in the 400 we have seen many of the paramilitary traits. But in none of them have we seen all (or even most) of the fascist traits at the same time. Even setting aside the fascist ideology and goals as quintessentially modern and an anachronism to apply in classical times, still none of the instances we have seen so far fits a very large number of other categories either. I can only believe that that is because fascism was the product of a particular historical epoch that brought those traits together in unique ways.
Consider what a genuine proto-fascist (or proto-proto-proto-fascist) movement in Athens under the circumstances we have discussed would have looked like. First and foremost, it would have required a completely different political configuration than the one that existed at the time. (Many thanks here to Donald Kagan for insights I have gleaned from him). In actual, historical Athens, the lower classes were the strongest supporters of empire and expansion and the upper classes its strongest opponents. Kagan suggests this was in large part because the lower classes were the main beneficiaries of the tribute it brought in, while the upper classes in subject states bore the main burden of payment. This, as well as Athens' tendency to support democracies, goes a long way toward explaining why it was the oligarchs who were most disposed to revolt. It also explains the Athenian upper classes' tendency to oppose the empire as a sort of expression of class solidarity with the oligarchs in subject cities. (Kagan also believes that their influence was strong enough to keep Athenian hegemony from becoming too oppressive and burdensome).
But for anything like real fascism to have occurred in Athens, we would have to imagine those positions reversed. In our imaginary Athens, roughly equivalent to Europe in the late 19th and early 20th centuries, the upper classes would be quite militaristic, and the middle classes (hoplites) strong imperialists, while the lower classes (sailors) would be most opposed to war out of an attempt to foster class solidarity across borders. There would also have to be a hard-core left-wing radical movement, not so numerous, but scary and of dubious loyalty. We would then have to imagine a young and charismatic Alcibiades firing up particularly young men of the hoplite class for the Sicilian expedition, with the stodgy, conservative Nicias somewhat reluctantly going along, and the sailors and their leaders somewhat reluctantly closing ranks lest their patriotism be questions. The mutilation of the herms would be blamed, not on an oligarchic conspiracy, but on left-wing radicals. Moral panic would sweep the city, with a crackdown vastly exceeding what was reasonably called for and, once again, the most of the sailors and their respectable, mainstream leaders would join in.
The Sicilian expedition would meet the disastrous end that it really did, in part (as was the case) because of Nicias' incompetent generalling. Athens would then accept peace on the terms it could get, which would naturally be bad ones. The government's domestic legitimacy would be undermined. The radical left would be strengthened and declare the whole war a mistake. The moderate left would agree, but refrain from actually saying so lest its patriotism be impugned. And Alcibiades would lead a populist movement of young super-hawks who had been the strongest supporters of the expedition, arguing that the only mistake was not hitting 'em hard enough, and that only that weak-kneed Nicias and those treasonous sailors were to blame. He would gain a strong following among the hoplites seeking to disenfranchise the sailors and build his own paramilitary to intimidate them.
As you can see, this attempt to translate the equivalents of early 20th Century Europe into the actual situation of Classical Athens just doesn't work. Fascism was the product of a unique time and circumstances that does not translate well into other contexts. But we can still follow its characteristics and see how they cluster together, and which movements in ancient times clustered most.