Can it apply to animals eating animals in the same group (example, dog eating fox) or does it have to be strictly the same species?

Some "groups" are more diverse than others, so defining cannibalism by that waters down search results. If a person wants to see two members of the same species eat one another, label it cannibalism; if someone wants to see less specific results, they can use eating/vore, duo or group, and specific species tags.

Well, vore's not my thing, but:

In my opinion: Cannibalism is not going to be something people will specifically blacklist. If they don't want to see it, they'll already have blacklisted vore. They'll use cannibalism to seek out specific images.

So, I'd say that if it looks pretty visually similar, then tag it. But that's my two cents.

It's kinda of a weird term when applied to anthros and doesn't really fit super nicely. Looking through the tag it's... hm. Not great. It gets tagged for hard vore involving multiple species, whitekitten stuff, characters being cooked, characters eating themselves, characters eating their eggs, mixed feral+anthro, mentions in text, Hannibal Lecter costumes... Not good.