Shaman spells without a "level" (like Petty, Rank 1, Rank 2, ecc...) are to be considered Petty Magic? If not, what's their "level"?

No ideas about this?

Why the question? What do you need their "level" for?

Because Chaos Shaman and Necromancer spells have Rank, while WAARGH Shaman spells don't.

But what do you actually need to know their rank for?

AFAIK, the only place rank is used is when determining how many advances it costs to buy the action card, and if you are using the optional rules in the GM tool kit, the difficulty for the P.C. to cast the spell.

In 2nd edition Shaman spells were relatively limited as I recall, and I personally assume those in 3rd ed will be similar, so in effect all WAARGH spells can be considered 1st rank, but unless you are using the optional rules in the GM toolkit, the actual rank won't affect play anyway. 2nd rank and beyond spells simply don't exist.

I also think giving Chaos and Necromantic magic "ranks", allows FFG to expand on this in the future and allows for Chaos P.C campaigns, again, just like 2nd Ed did. Personally, I don't like the concept, but I think a lot of other people did.

For "ranked" NPC WAARG shamans.

But since there are so few shaman spells they can simply have all.

Remember that rank on the card isn't strictly a limitation anyway, so a rank 1 NPC could use a rank 2 card if you wanted.