Is it OK to use a #14 wire pigtailed to #12 ground wire inside the outlet box? I was told once if it is only a few inches to the gound screw it would be OK. It make it easier to tuck the wires into the box.

No, everything on the circuit must be #12 if it is a 20A circuit.
No reason to downsize.

You can use ground pigtails that you can purchase. Some are stranded and some are solid. Clips, forks and ground screws are common options.