I have the PAL Canon XL1s. I was reading some article, and it said that's it best to shoot at shutter 1/50 (for PAL version), for film look.
Now, if I wanna use the Iris wide open (f/1.6 or so), to give shallow DOF, how can i keep the shutter the same? I know I could use the built-in ND filter in the lens. but that wouldn't be enough for outdoor shooting. Should I stack more ND filters on the lens?

Yes, use ND filters and you can add a polarizer if you want the polarizer effect. You cut the light by about 1 and 1/2 stops with a polarizer. Ideally to minimize the effects of diffraction a lens should be used about 2 stops down from maximum aperture. If the lens has a maximum aperture of F2, F4 would be the sweet spot. If you need the smallest DOF use F2.