The custom is a bit small. It needs to be a little bigger to see more detail, but not bad. The shading on the stomach should be reversed, because there should be a dark shadow under the head, not a light one. Just makes it look odd.
The pre-evo is pretty good as well. The front leg looks off. It is slanted up towards the body and then all of a sudden gets straight. The outline could use some work as well. In places, it looks like you used the darker shade of blue, and then you used black in places.
The fusion is good. Of course not too complicated, but good.
The "Mecha" Tyranitar just looks like a gray recolor to me. :\ Doesn't look very mecha.