I've been rewatching M.I.High for the nostalgia, and I just came up with a theory. What if Della/Sally, the little girl from the Agent X episode, is actually one of Zoe and Keri's sisters? It literally all matches up when you think about it. Della's been brought up by SKUL to work as an agent for them her whole life, just like Zoe. Della's 10 when we meet her, and the episode that introduces Zoe, who's 14, aired four years later, so the ages match up exactly. Della's real name turns out to be Sally, which could have been a code that was tattoed on her arm just like Zoe and Keri's names (SAL1 or maybe SALIX if that's where her Agent X persona comes from). It all makes sense!
Also another theory I have is that maybe it was meeting Della/Sally that persuaded Zoe to leave to track down her sisters. Because I like to think that Sally would have kept in touch with Oscar a bit. And maybe after the series 6 finale Frank called on Oscar and the rest of the old gang to get a bit of extra protection for Zoe and that's when oscar joined the dots and realised Sally might be Zoe's sister and introduced the two of them, and Zoe left to get to know Sally better. Idk, I just love the idea that somewhere offscreen, Zoe and Sally went off to have sister adventures together!