Guppies and tetras, maybe - if they're bumped into at night. It shouldn't bother the cories. What kind of gobies?
They (well at least mine) surface feed. Their bottom dwellers but mine swims upside-down at the top of the waterline and eats floating pellets. Synodontis is a genus of upsidedown catfish, its a trademark so the fish is not sick or anything. Just what it naturally does.
In that picture the fish looks absolutely terrified. Are you sure it's not really yours and has actually just met your killer Plec?