The thing is each fictional book movie etc. is an interpretation of a general idea thats why they vary so much. It always someone elses vision based on a generalized idea.
As for zombies eating zombies, it seems like all they are left with once reanimated is instinct giving them the ability to tell their own kind from others. I assume once they have eaten all the humans they would resort to cannibalism, sort of a Donner party situation slowly eating all the zombies until they finally deplete all food sources and dwindle away.
Most animals are too fast for zombies to catch thats why you seldom see them caught or eaten by zombies. Not to mention that instinctively they would avoid zombies as much as possible just as they do humans.
now that I have answered your questions to the best of my abilities I have one of my own. I know in some movies they do address the animals turning into zombies issue (such as resident evil) but few animals eat rancid meat and once they have would they in turn have a craving for living flesh turning the rest of the animals into flesh craving zombie beasts? Or would they continue to scavenge?, because it seams that being reanimated leaves us with our very basic instincts only doing what we need to survive.
I realize that I/we are getting way too deep into this and its all theory based on fiction but I like to see what other people think, and why the hell not Im having a good time with it.