The concept of Stage is irrelevant if you're loading a movie into it: Stageonly applys to the main timeline.
If you want to scale the movieclip to the size of the Stage try this:
Code:
myClip_mc._x = myClip_mc._y = 0;
myClip_mc._width = Stage.width;
myClip_mc._height = Stage.height;
You'll need to publish for at least flash player 6.
cheers.