Thanks so much for the feedback!

I definitely agree that these sprites look a bit too solid for slimy things.

I probably should have mentioned in the description but some of the requirements I had for these sprites were the (relatively) small sprite size and adherence to the NES' colour palette, which makes it a bit harder to maintain consistent shading over multiple color schemes.

But thanks for the gallery link, I look forward to trying some of those techniques out.

I'm not getting their full sliminess, I know it's difficult because I've tried and failed to do something similar, I think you need some darker elemens for where the slime is on the floor and some more highlights.

Look here for a weekly challenge for slime:

http://www.pixeljoint.com/pixels/challenge_item.asp?id=1207