I tried your suggestion, and if I use 3-4 layered spheres I get enough variation in the star movement so that it looks very cool.
Granted I'm not sure if it's 100% realistic, but hopefully the foreground action will be distraction enough.
I still have to figure out some way to do planets so that stars aren't seen in front of or behind them. I apologize that I'm pretty uninformed when it comes to this painting and texturing stuff, but any ideas would help. Maybe a planet-only layer, where the rest is transparent? If that would work...how do I do it?
Here's my big problem though...my painted 2D space background image is 640 x 480. The spheres that I map that on to are quite huge, as they need to be large enough so that the universe looks infinite. My issue is that, this 640x480 image is stretched to cover the large sphere, and is by far no longer crisp or even remotely attractive. The stars begin to look like marshmallows or cotton rather than sharp points of light.
So, I need a way to get around this image-stretching. My best guess for how to handle this is by painting directly onto the universe sphere. This means, I have to paint a lot more obviously, but then there would be no stretching.
Maybe I should just stick with 2d planes and restrict my shots to pan/dolly without rotations?