I suppose I can only offer my opinion, but I'm not all that good at splices myself.
I don't care for the whole top row. They're ok, they just don't really appeal to me. The Seviper/Arbok one I do like, but the yellow scales on the tail just look glued on. And I don't really care for the Charizard/Feralligator one. Don't get me wrong, they're well done, I just don't have much taste for the whole "What would this pokemon look like with a few new doo-dads and colors" thing. I admit it's hypocritical, I do splices myself but I go out of my way to try and make them look different.
The Pokeballs... I dunno, I don't really care for those either, they just seem... like most fusions : tacking on unneeded parts.
Now for the REAL reason I replied: the customs.
The Key, it's ok for a design, but it doesn't really match the style at all. Too many colors, too little contrast, and the shape just seems a little undefined. Round out those edges like an Unown- actually Unown would be a very good reference for that pokemon, just look at how it's shaded and how it plays with lights and shadows.
The next one, the Grass Snake, needs a bit more smoothing out and probably could do with a bit shinier of a body too. Look at the three snakes in pokemon for a reference there for the style. Again, too many similar colors.
The Fish... has outline problems as well as shading problems. Look at a the fish pokemon and look at how they're shaded and defined, you may get some inspiration if you want to return to that scratch.
The next two suffer the same problems as the Grass Snake, as well as being a bit too tapered off on their aft end.
That last one just... kinda has me at a loss? Is it fire, or ghostly smoke? Either way, looking at the effects and getting a good reference would help loads. As well as looking at the creatures like Tododile and maybe even the Sandshrew family would give you some good references for that creature.
Sorry if I hurt your feelings being overcritical, but again, it's all my opinion, you're free to disregard it and completely ignore my post.