I bought a Phantom 3 Standard but the mobile device holder is too small for my tablet(8"). Can I adapt the Phantom 3 Pro device holder ? (or Phantom 2 )

The Inspire 1 holder is better suited...metal bracket is less wobbly.  It can be done, there are threads with pics available.

There are also aftermarket ones online fleebay and others.

i dont think you can mount an inspire on the p3s controller.

I bouhgt a $12 holder that is shaped more like the PS3 holder but with an inspire holder on the end