I have several fics that spawned from prompts, some of which are more detailed than others, with "my most piesed possession" and "you're full of something (but it ain't love—right?)" on the more detailed end prompt-wise. I expect these are not collaborations? But something like "some hearts have the stars on their side" or "Cover Up", both of which I wrote by myself as a sequel to someone else's fic, I expect those are collaborations for this purpose? (If not, "Cover Up" at least needs to be marked safe, I already annoyed one of the creative forces behind the fic it's a sequel to by writing it without checking with her first...)

If the series already exists but only has one work in, will marking that safe be my safe series or one of my three safe works?

I'm going to guess the answer to this is no, but can I safe-fandom out everything in one fandom except the crossovers? Because filter my fics to include only Ancient Greek Religion and Lore and there's fifteen, twelve of which I don't want remixed and the other three are also tagged with song fandoms because I wrote them for jukebox_fest, and I would argue that they are primarily fics of the song fandoms and I am cool with those three being remixable. (If those three have to be marked safe in order to keep the other twelve safe, that's fine.)