Thanks!
I started by making Pok?mon fusions, that's how I learned the basics. Basically I'm self-taught. But there are some decent pixel art tutorials on deviantART if you care do some searching.
Of course, could be your style, but your outlines blend in quite a bit. For example the gastrodon in the first post, the outline is invisible against the flat colours, and in sprites it usually best to have a distinction. Of course, its my no means mandatory but it might be interesting to try.
You should try having more contrast in your shades too, I didn't know there was any shading in that sylveon until I had a closer look at it.
It depends on the art style I choose. As you can see I can variate my style heavily as I please. Flareon has much contrast, the lion cub has strong outlines, etc.
Sylveon doesn't have shading. I mean it does (in practice), but it's only there to not make it look completely flat. It's not meant to have noticeable shading.
Btw. I don't understand what you mean with the Gastrodon. There are no outlines at some points to ephatize the fact that it has been eaten from those spots if you mean that (salad snake, they eat each other). The fusion / Pok?mon colour choices are Gamefreak's choices anyway. All fusions use existing palettes.