You will enjoy the Zeiss 21mm f/2.8, it is a lovely landscape lens.
I use CPLs a lot (52 and 72mm), but have not yet settled on a large CPL for the Zeiss, either a 82mm CPL or a 105mm CPL that fits in front of the Lee system. CPL on a 21mm FF lens will give you weird skies if you have sky all across your photo, but there are plenty of other reasons to use CPLs on ultra wide lenses - waterfall/stream reflections, improvement of foliage saturation being the two pertinent uses in my local (Ozarks) landscapes.
I settled on using the Lee holder, and bought a Big Stopper 10-stop ND filter, a 2 stop soft graduated ND filter, and a 3 stop reverse graduated ND filter (sunsets and sunrises). I am going to be using the filters on 4 x 5 film, and on a Sigma Foveon sensor camera, which has less dynamic range than a standard Bayer CFA sensor on a DSLR.
, they are very helpful and will answer your questions, in fact the owner is a chatty guy. 2filter may have the best access to Lee filters in the US, they seem to have them in stock far more often than at B and H.