Best Practice

Trying to get a handle on the proper method of developing Flash design.

I want to build a simple site that starts out with a small box in the upper left hand corner of the screen. The box expands to a larger size via animation. On the right hand of the screen there is another box that expands downward, this will house the navigation buttons.

I know how to do all the animation for this, but am wondering if I am making it more difficult than need be.

Would it be best to create the two boxes and buttons as independent movies and than place on a layer? If I go this approach it is difficult to work in that the animation happens within the independent movie and not on the layer I have placed them on - makes positioning items harder.

Should I place all items in one movie across multiple layers? If go this approach I fear I will build a layered mess, and become increasingly difficult to build upon.

New to Flash and just wanting to make sure I am not developing bad habits. I understand the basics of drawing and tweening, just want to have a better grip on how to group symbols, layers, etc. in a more efficient manner.

All help appreciated!
Thanks,
Kerry