As far as possible, try not to stack filters as quality will degrade, and at times, there will be vignetting. Unless of course you are talking about using CP and multiple ND grads at the same time, then I guess that's the only way to go about it.
You can, but you should not. Especially with a UV and a CP, or a UV and a color filter. Each additional filter and element you introduce increases the risk of image degradation. The light degrades everytime it cross a glass/air interface. Depending on the construction of the lens, the light will also bounce around the insides of the lens.
However there are situations that may warrant stacking up lenses. Example: CP and a ND filter to create a more saturated blue sky and to reduce the contrast between sky and foreground. Another example for B&W photography. Red filter and a CP to make the sky pitch black.
1 stop is a measure of light. Your image won't "under expose easily". Just that you have to use either a slower shutter speed (possibility of hand-shake) or a wider aperture (loss of DOF).
As the others have said, try not to stack filters. Could have undesirable results. Also if u use cheap filters beware of "ghosting effects" when u shoot at night. Will happen if your scene has bright spots of light. If u're affected just remove the filter before shooting, or buy the more ex. HMC filters.
i stacked hoya HMC UV + Hoya single coated CPL on my tamron 28-75 whilst shooting sunset in sentosa yesterday. one particular shot had a greenish BLOB on it.
but the rest were ok. not sure if it was cos i stacked fitlers.
oh....i also stacked manually, without step down ring. ;p to use my 67mm CPL on kit lens, i focused, palced the CPL infront of my kit lens, then turned and held it there. hehe. anyone knows if this method is ok?
Yup the greenish blob's probably caused by the filters. Heard that its because light reflects off the surface of the sensor, but instead of passing out of the lens it reflects off the filter and back onto the sensor to be captured. Sort of an internal relfection.
Thats why you don't see such reflections through your viewfinder when framing. Happens when you shot covers very bright spots of lights in a relatively dark background, ie sunsets, night scenes in the city.