Glass has been used for other types of animation, apart from just cutouts. I know Rex the Runt does something with glass layers.

I'm not entirely sure, but I think most of the questions depend on what you can get and preference. As long as: the glass doesn't have cracks or scratches or anything that a camera will pick up, doesn't distort light coming through it (like a prism), doesn't collapse fromt he weight, etc.
Distance between them (same light issues apply from above) - as long as you can get ur hands or a tool in to change the movement, then it's ok.

I'm sure someone will have more definite answers than mine.

Hi Jesse-

Thanks for your input. I guess I'll just do a few experiments.

While I was waiting for a response, I reran the video and tried to figure out the spacing.  The glass appears to be 3/16" - 1/4" thick.  There are actually 4 layers of glass.  I think it looks like this from the bottom up.  Table (Background) + 4" spacers + 1/4" Glass + 8" - 12" spacers + 1/4" Glass + smaller Glass sitting on top of second glass + 4" Spacer + 1/4" Glass + Camera