Cokin P filter holder & step up rings

I'm going to be buying a Hitech 85 Soft ND Grad set and a Cokin P Filter Holder.

At the moment, I have step up rings for 2 of my lenses to allow them to use my 67mm CPL. Would it be okay for me to buy a 67mm Cokin adaptor and use it with each of my step up rings instead of buying a Cokin adaptor for each lens? Or will that produce lots of vignetting?

I've been reading around but couldn't find much info on this.

Thanks.

I assume the 67 mm is your 17-50? If so, and you're not getting vignetting with the others now, you should continue to be safe, although the Cokin P adapters are pretty inexpensive to begin with.

Yup, that'd be the 17-50. No vignetting when I use my screw-on CPL with my smaller diameter lenses and step-up rings.
I'm mainly concerned about the extra depth of the Cokin adaptor ring screwed into a step-up ring, then having the Cokin filter holder attached. It seems like a lot of things to attach.
I assume the Cokin adaptor rings are about the same depth as the average step-up or step-down ring?

The adapter ring proper adds practically nothing to the depth of the holder as it is recessed into the holder. If you're concerned (and the chief danger will actually be with the 17-50 at 17), get the WA Cokin holder - the regular one has 3 filter slots, the wide has 1 (and a thin one for gels).