I don't think the bone on the Six Tail Fox is armor. I think the bones are actually the Fox Demon's internal skeleton.
Underneath the skeleton is the spirit/charkra of the fox (for tails 1 through 5).
If Naruto goes to 7, 8 and 9 tails, the fox will grow bigger, and soon you'll have muscles covering the skeleton and finally hide and fur to make the transformation complete.
