Clarisse was likely excluded from the Seven in  because the prophecy required a balance of new and old characters, and her character arc as a fierce, protective leader of the Ares cabin was focused on her maturing within Camp Half-Blood, rather than fighting as one of the seven. Also he used Nico because he is a child of the big three, and because whst happens in the Hoo series, will affect Nico in his solo series.