The Mixels have one serious flaw in their leg designs in that the part with the ball socket is the hip joint. If you put the part with the ball joint as the hip, it will give you better forward/backward articulation. You should also try to do this with the shoulder.
Also, it looks like the body is built by bricks stacked on eachother right-side-up, correct? Unless you have sideways parts on the front or back to better support them, they'll probably fall apart. (Though at the small scale you're building in, it might be OK) I would suggest you re-work the design of the body to be flat plates facing forward with a sideways bracket joint to attach the head (I'll try to send pics of what I mean later).