I think it's too easy to say that someone "ripped something off" or that he is "re-making" someone else's work.
Sharpe and Gaunt, to name an example, share very few qualities other than being imaginative commanders who care about their soldiers despite being thrust in a vicious war. Even fewer of those qualities are anything more than tangential connections (e.g., both Gaunt and Sharpe have a likable second-in-command whose death drives home the sense of loss to the reader).
And where the Iron Snakes of Ithaca are concerned, virtually the only real link between them and "300" is that the Spartans were a Greek people and Abnett looked to mythic Greece for inspiration. There is, literally, nothing explicitly Spartan about the Iron Snakes.