well, Ive had a breeding pair of convicts kill a red devil & a carpintis in a 125, and take over the whole tank after doing so. 2 weeks ago, in my 265 I had a male convict and a female Honduran red point pair up and they ganged up on every fish in the tank, took scales off an argentea, grammodes and a cyano. Before I could get the male con out, they had corralled all 22 other fish into 1/3rd of my 265.
Ive had this happen on almost every occasion Ive had breeding pairs of convicts which is why I keep them alone when they do spawn. If you want a breeding pair that isnt as aggressive, try the Honduran red points. I have a breeding pair in the 265 as well and they simply keep everyone away from their area in the tank, about a 1 ft radius.
More than likely, a GT or JD or any other single fish in a 55 with a breeding pair of convicts over 3 or so inches, is gonna get worked over, stressed or the GT or JD will kill the convicts for messing with them. Ive never found a balance in a tank with breeding convicts, even up to a 265 gallon.