if by downsize you mean change the image resolution, then no, that is not pixel art. well, downsizing in any meaning of the word is pretty much cheating in pixel art, that is, unless you are getting rid of unnecessary colors. You seem to have a solid base, though i think there may be one (or two) too many shades of gray. Try limiting your palette to only 3-4 shades/tints per base color for a piece this small. If you do greyscale for the entire image, you will find it harder to make it into a color piece because you cannot simply convert it to color with the fill bucket, you'll have to go over it again, that is unless you want a completely mono-tone image.
I'd say go straight for color, or at least block off the color areas, and then refine it. You can always edit the palette from there, it's a much simpler process.
You've got the hard part out of the way, the lineart, at least it's behind you.