Yes, you can stack a polarizer and an ND filter on a Canon 18-55mm kit lens. The main concerns are image quality and handling rather than compatibility.

Things to watch for:

• vignetting: stacking filters can darken the corners, especially at the wide end of the lens
• flare/internal reflections: extra glass surfaces can increase reflections and reduce contrast
• color cast: some ND filters, especially cheaper ones, may shift color, and this can be more obvious when combined with a polarizer
• usability: on some Canon 18-55mm versions, the front element rotates during focusing, which makes adjusting a polarizer more awkward

So yes, it’s possible, but use stacked filters only when you actually need both effects. If you buy them, try to get decent-quality filters and test them at the widest focal length to check for corner darkening and color shift.