06-13-2014 10:22 PM
You can but I doubt it's a good idea. Many filters soften your photos, and some lenses just don't seem to like having a filter at all so I'd recommend removing a protective filter if you have one before installing the CPL or any other filter.
06-14-2014 08:58 PM
You can combine, but since a UV filter is technically redundant (there's one inside the camera already), there's little point in combining a front-threaded UV filter with anything else.
I have actually combined ND filters (stacks) and combined ND with CPL. If you do this, you probably want to put the CPL on the front since you have to rotate it to tune the polarization.
You can also stack slide-in type filters such as gradient neutral densities and those commonly are stacked.
HOWEVER... be aware that each filter has a nice flat reflective surface and this can cause "ghosting" to show up in your images. You always want to get multi-coated anti-reflective filters to reduce the ghosting problems.