Five Figure Fanwork Exchange

I just added a nomination for Adam Goodman & Julia Upjohn under "Cat Among the Pigeons - Agatha Christie" but I didn't notice that "Poirot - Agatha Christie" was already in the tagset and is synned to it, so AO3 automatically (& irreversibly) changed the fandom to that. You mentioned that you'd generally prefer to avoid subset fandoms, so do I need to keep this under the larger Poirot umbrella, or would it be possible to have Cat Among the Pigeons as a separate fandom? I'm not asking for any characters who appear outside this book (and all my mentions of Poirot himself in my prompts are "but what if he wasn't in the book at all").