What snookie said, if either of the spp forms a pair there will be agro, two male dempseys together may be ok, as you say trial and error.
You can definately add more fish, as long as your tank can hold them, the pleco shouldnt get any trouble from them either.
You could try some other fish i have clown loaches and a featherfin syno with my big JD female and had a adult female convict in there too. Ive never found JD's to be that agro unless breeding, convicts similar actually but maybe more teritorial.
But yeah if there is any pairs in the tank there will be issues.