I haven't seen the show, so my comments are based only on the reference pic you included in your post.
Your pixels are two different sizes, 22 pix for the top row, 33 pix for the lower row.
None of the creatures in the source pic have eyebrows, several of your creations do.
Only one of the ponies has closed eyes (the white unicorn) and it has some sort of eyeshadow. This works well to keep the shape and size of the eye visible while letting the dark eyelid control the expression. Unless this character is an exception, consider adding this feature to your icons. It serves a similar purpose to eyebrows (defining the eye socket shape).
In the source image, the heads appear to be 5 units high. One unit for the nose/mouth, two units for the eyes, one for the top of the head (which may be covered by hair) and one for the hair above the skull. There is some variation, but this seems to be a reasonable average.
Simply scaling the faces might not give you the desired results, however. The nose and mouth are crammed into a small space, and shrinking this will make the faces less readable. You might be able to squeeze one pixel out of this space, but I'm not sure you can get more than one pixel from here. Losing the nostril would make the noses resemble beaks, so I think that becomes one of the limiting factors.
Keeping the lower part of the face means reducing the rest of the face a bit more. With a bottom of jaw to bottom of eye distance of about 6 pixels, that leaves 26 for a 32 pixel face; 13 for the eyes, 13 for the head and hair, which should be plenty.
If you've only got 22 pixels for the icons then you would have to shave everything down. Lose the nostril, shrink all of the features, possibly use a couple more colors in order not to distort the nose shape too badly.
In the source, the top of the skull is a bit higher than the base of the ear. In some of your images this is not the case. I think lowering the skull makes the ears look more like horns and less like ears, and should be avoided.
In the source, the mouth is never wider than the space between the eyes. You use a wider mouth for more expression. It's a trade between adherence to the source and your goals for expression. I recommend trying both and see what you like better.
Hope this helps,
Tourist