Gonna echo what Fickludd said. Use dark outlines only when you absolutely need them to distinguish forms; otherwise they just take up precious space and increase clutter. Try using differences in value to do the same job.
I did some messy edits on the leftmost four sprites. Many of your colors were saturated shades of orange, which isn't bad in and of itself, but your palette could use more cooler blues and purples to add interest. Also, recycle colors throughout the sprites. There were several isolated clusters of colors that only appeared once, and you could either eliminate those color or come up with ways to reuse them in funky combinations.
Finally, consider giving yourself some breathing room in terms of your characters' proportions. You're trying to pack a lot of detail into small areas; it might be easier to just make them bigger -- for example, the heads on all of these guys are pretty small.
Hope that helps!
