Never copy from somebody else's art. Copy from real world objects. If you do fan art, don't reference it to the point where it's either just a direct copy, or the style itself was copied. Be original where you can and trust me, you will get a lot more points for it. If you have trouble anywhere in some sort of object or anatomy, then it's time to refer to real world objects or people and draw those until it makes sense.
Put it this way, would you rather be an imitator or a unique artist? The only thing copying another artwork directly is going to teach you, is how to draw like somebody else. It's not very valuable and you can learn so much more if you just copy from real world objects instead, and keep your fan art original and unique if you even choose to make fan art.
At least you showed us what you did though. I always hate it when people copy a drawing and then try to hide that the other drawing existed, to try to make people thing they made it up. Copying real people or objects is OK, because those weren't drawn before, so the person actually learns something.
The other drawings are pretty nice, but to be honest, much like Mr Cookie's, they are rather simple. However, you guys do at least know how to keep things with that clean look, which means you know your limits and can make good use of what you can do. Just don't be afraid to experiment and risk having an ugly drawing, yet more complicated. Otherwise, all you'll ever make are these simple drawings. Again, they aren't bad, but they are sorta just there. They do have an appeal, but it's not enough to get me interested.
