Well, you can turn on scoring and play through the game, I'm pretty sure you'll also get the regular campaign achievements with the score achievements if you work at it.
Though, I can't test this theory as I won't actually be able to play the game until tonight.
Though, I don't think you have to find skulls in the Meta-Game. I think they're just like Halo 2...only you can turn the effect on and off right before a Meta-Game starts.
I'm looking forward to using the skulls that make enemies dodge faster and make their armor even more resistant.