Yeah Cons will tear up firemouths, I learned that one the hard way not to long ago and it led to euthanization. Convicts are happier by themselves, or at least mine is.
I've been using feeder guppies as dither fish and they seem to work real well. I have one male Con in a 30 gal with a pictus cat and add feeder guppies when necessary. The pictus eats em at night but the Convict seems happy just terrorizing them. He doesn't chase em or attack em, he'll just let em know he's there. So at least from my point I would say its prolly safe to put in some fast, small, topwater fish like zebra danios, something small enough to not make them feel threatened but fast enough to get their attention and keep them active.