Trust me, (please don't make me drag out all those old textbooks!) the standard plural ending for a Greek noun ending in os is oi. Dynatos (a mighty one, a lord, etc) and Dynatoi. Doruphoros (a spear carrier) and Doruphoroi. Akritos (a Byzantine border noble) and Akritoi. Kataphractos (an armored horseman) and Kataphractoi. Psilos (a skirmisher) and Psiloi.

The reason the plural of "ethos" doesn't show many entries in standard encyclopedias and dictionaries, is that it isn't used in common English. We speak of Billie the Bard's Ethos, or whatever. But, just like the idea of "One man, one vote!" we have the parallel concept of "One man, one Ethos!" And, therefore, "Many men, many votes", and "Many men, many Ethoi".