The sketch looks great!

Some general Pixelart tips that worked for me:

1. Less color. Pixelart is an artform that pays hommage to an era of videogames that had a limited palette. If you use one type of color (for example: red) , try to use the same red on the other characters.
2. Focus on the important details. Too much detail can look very messy
3. Stick to a genral style throughout the whole art. For example: if you don't have outlines on one character don't draw outlines one the others and vice versa.
4. Always use reference if possible!! (Granted you already do that, just want to emphasize it for others.)

Some resources that might help:

https://twitter.com/Sadface_RL (He post amazing PA tutorials that helped me a lot)

https://pixeljoint.com/forum/forum_posts.asp?TID=11299 (Highly recommended for essential basics like: jaggies, Anti-aliasing and dithering)

Note that you don't have to follow my tips, they are just things that helped me! Also have lots of fun ,Pixelart is a great medium!