Electrical – My outlets have ground wires twisted together and bonded to the metal box, but not the outlet. Is this ok

Read inside the box. If you install 14 AWG conductors to the new box, instead of 12 AWG conductors. You'd need at least a 22 cu.in. Box, or a 3" x 2" x 3 1/2" device box.

• 8 for current carrying conductors.
• 1 for grounding conductors.
• 2 for device fill.

11 total, times 2.00 cu.in. for 14 AWG conductors = 22 cu.in.

However, since you're using 12 AWG, the calculation looks like this...

• 6 for 14 AWG conductors x 2.00 cu.in. (12 cu.in).
• 2 for 12 AWG conductors x 2.25 cu.in. (4.5 cu.in).
• 1 for grounding conductors x 2.25 cu. in. (2.25 cu.in.).
• 2 for device fill x 2.25 cu.in. (4.5 cu.in.).

Total = 23.25 cu.in.

For more information on box fill, check out this answer

As for connecting the new wire. There should be no problem attaching it to the receptacle. However, since you're using 12 AWG conductors, you'll have to use the screw terminals.

If you want to do pigtails, you can. I know some guys don't like the backstab connections, so if you want to avoid using them you can do pigtails. Ideal 76B® Red Wire-nut® Wire Connectors are rated for 2 to 5 #14 conductors, but check with the manufacturer of the connectors you're using. Again, if you're going to do this, you'll want to use #14 conductors instead of the #12 you've installed.

Short answer, you do want a ground conductor between the main panel and subpanel (not just a ground rod at the subpanel) and you only want the neutral bonded to the ground at the main panel.

The ground rod alone is not sufficient - although there would be a path through ground back to the main building's ground, that is not a low impedance path, it wouldn't be low enough to clear a fault, not safe, etc.

The ground conductor could be a metal conduit from panel to panel provided it's installed properly so that it creates a continuous path back to the main panel.