Vagueness gives you more latitude. If you want to have pokémon eat other pokémon, do it. If you don't, then you don't have to. You mentioned yourself that we basically never see normal animals in the pokémon world; this implies that if they exist, they are scarce (at least the kinds that one would notice, anyway), or it's related to what Xanthine mentioned earlier--because the focus of the characters in various canons is so much on pokémon, they ignore the normal animals in favor of the more specialer ones, and so don't notice the mundane sorts.On another note, if my memory doesn't fail me (again), there has been multiple mentions about Zubat/Golbat sucking blood of other Pokémon and even humans. And Ekans eats the eggs of Pidgey or something. Oh, and Feraligatr's entry says this: "When it bites with its massive and powerful jaws, it shakes its head and savagely tears its victim up." (from Bulbapedia) So, it does eat meat! But what meat? Do they have to be so vague about this?
Well, duh, kids would get traumatized if ther Feraligatr was told to eat Pikachus or something...
It's up to you, really. Either interpretation can work, so long as you back it appropriately. There are other animals in canon, as Astinus pointed out; what you want to do with that is your call, as the issue is something that just isn't explored.