Questions on anime physics are, in my opinion, fine. For instance, asking how in-universe mechanics work can generate some perfectly valid questions. It's typically impossible to tell a priori whether a question like this will be answerable with in-universe information, or if the only possible answers will be speculative. I occasionally see questions like this on Scifi, which we tend to use as a role-model site, and they can generate good answers. (They can also generate low-quality speculative answers, but those typically get downvoted and replaced by better answers.)
However, I don't see much use for the tag anime-physics. I'm not sure who benefits from it. My feeling is that the users who will understand these sorts of things best are the users who are especially familiar with the series; there are no experts in anime-physics as far as I can tell. I also don't see it being terribly useful for searching or for classification purposes.