I have some info that might or might not be useful. So changing sprites for all the other bosses works fine. But it doesn't work for Fang, Rosy and M.Sonic. And there's a single thing that all of the objects have in common. They all are based off playable characters. And it seems that both the sprites in srb2.pk3 AND characters.pk3 have the same sprites. Like with Fang for example:
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The green one is from characters.pk3, and the colored one is from srb2.pk3. Yet there's absolutely no reason for that sprite to be present in the characters.pk3 (The folder for playable variants) seeing how Fang never uses this sprite in his playable version. (And yes, I already tried replacing sprites in both folders... it didn't work... but maybe y'all will have more luck... :C)
And I have an "OBJECTION!" moment because the Rosy sprite folder in srb2.pk3... actually uses the OLD sprites! So this "proves"(?) my theory of Srb2 using the player sprites for cameo characters. But I don't really know what to do with this now...
I mean in terms of gamedev, then it would make sense to do this to safe space (And maybe sprite slots?), but in terms of an open-source project that's free for everyone to edit, then it just makes it harder to achieve something that was easy back then. And the sprites folder is right there free to use... :C
And I also tried making two pk3's named "srb2" and "characters" exactly the same as the main game data, and I tried doing it that way. Yet it still didn't work... This has to be some anti-modding scheme fr fr...

(At this point I just want to ask someone from STJR to know how that works and if there's even a possible way to replace these sprites... :C I bet that if I would even modify this game to it's CORE... the only thing unchanged would be these sprites... -_- But the character sprite theory might have something to it...)