If they are jack dempseys, and you are able to upgrade to larger 4' foot tank the convicts and j/d would do well together. But you would need to upgrade in about 3 to 5 months depending on diffrent things, primarily size, but agrresion as well.
I have a red devil he is about 4 1/2" he is so bad i just took him out of the tank, he killed my pacu, tore up my oscar and beat the crap out of my red tail catfish. Time for him to go, anybody wanna trade something? or adopt him? Im in LI New York.