It's probably just me but the Nidos always occurred to me as omnivores. The most telling of these would be Nidoking's pokedex entry for red and blue. Another thing is, Nidorina and Nidorino's fangs. One could argue that it's just for self defence but Nidoran buck teeth would still be effective. I've been bitten by two different animals with prominent incisors and it's always broke the skin.
That and, considering they're packing incredibly deadly venom, Nidos would make highly effective predators. They'd probably just eat whatever they could kill, which if Nidoran female's most recent dex entry implies, they could kill an awful lot.