Protective earth bonding for multiple AC-DC PSUs
You'll want the protective earth connected to all the other protective earth terminals (on your power supply units) as well as to all exposed metalwork. The power supply should have its case bonded to protective earth already, but an extra connection point directly to your metal plate would be advisable.

As for DC ground, you don't need to connect that to earth ground, and it's probably best not to connect them yourself, as the PSU should have an appropriate circuit to connect the two grounds loosely while suppressing EMI. Connect all the DC grounds together, separately from the earth grounds.