digital - Do I need a slim or normal polarized filter for a wide angle lens?

I have a Canon T3i body and I recently bought a Tokina 11-16mm wide angle lens for landscape and astro-photography. I am looking to buy a polarizing filter, but I'm not sure if I should get the slim or the normal filter? I might need to attach another filter to it, so will it be stackable or not? I'm looking at the Hoya filter as it is within my budget.

Also, is the Cokin filter good for quality pics? I'm also considering getting the Cokin ND cheap gradual color filters, are they worth buying? Should I get the Cokin or any other ND filter type? Can you please recommend one for me?

I'm interested in landscape, waterfalls and so on.