You'll have an easier time developing your sketches into pixel art if you start with pixel tools, especially if you're working at the final size. Starting with clusters and then refining them saves you the work of going from idea -> clusters -> polish.
An example of a similar sketch I did for someone a while back:
This one doesn't have shading, but you get the idea - and shading would be easy to add in. It lacks pixel-polish, but it conveys the idea, and it would be easy to start refining the shapes from this point without having to redraw them first.
I am reluctant to critique these because I don't know which problems are your actual decisions, are which are just limitations of using non-pixel art tools at such a small size.
The only problem I know is your decision is the poor contrast. Do not fear high contrast, especially at small sizes. The smaller the area taken up by a colour, the less the eye is able to distinguish that colour from its surroundings. Higher contrast compensates for this, and makes the piece look less dull.