I think you will find that you can only get one filter in front of the lens before you start to vignette, and that should be a thin design. The 28mm is 72mm front threaded and it will barely allow a single 72mm filter before you corner vignette, and this is before you shift. The 32mm @ 86mm may behave better, but I doubt you will get 2x stacked and be able to shift much past 5mm before you optically vignette. The 86mm will not work well with the Lee system either since Lee refuses to make a 86mm wide angle ring, and the standard ring will vignette on the 32mm.
My main setup is the Lee wide angle hood, with the 105mm B+W CL-PL, and then I use the Lee glass ND filters or a CF on the lens.
If you purchase the CF, for this lens, you go from 86mm to 105mm. The CF in itself will act as ND as it drops the exposure 2.5 stops. You can then add a 105mm CL-PL but again, finding a thin design at 105mm may be hard to do.
As for brands, I agree with Joe on the brands he mentioned. I have B+W, and the Tiffen filters, (ones with the white water glass). Several have IR blocking built in.
Congratulations on the 32mm Rodenstock, excellent lens indeed. Be aware that this lens is a bit delicate and can have problems where the copal shutter is mounted. It's not recommend to carry the lens mounted on the camera on a tripod.
Paul