Part of the challenge with G9 nail morphs is that they aren't separated from the fingers. 

Looking at the image below, the only polygons that are not affixed to the finger are the highlighted ones, which can be morphed without affecting the finger.  That's probably what is causing the stretching.

The edges highligted in red are attached to the finger and can't be moved without affecting the shape of the finger.

The green ones that are ONLY on the TOP of the fingernail *may* be able to be morphed but you'd have to be careful to make sure that you don't select any of the red edges on the side of the fingernail. 

The other alternative would be to make ten nail smart props.