You can breed them in a 20 long if you don't want any other tankmates. They are versatile and breed in a wide array of pH levels, to be safe aim for between 7.0-8.2. Temp 76-83, on the higher end when breeding is desired. Nitrates, when, just keep them to the lowest level possible, and you certainly don't want measureable nitrites.
I wouldn't breed them with other tankmates in anything smaller than a 55. Jack Dempseys, Jewel Cichlids, Nics, Firemouths and Salvinis are all common cichlids that have been house with convicts with varying degrees of success. They all grow larger than Convicts, but jewels are closet in size. Jewels and Salvinis are just about as fiece as convicts, give or take some degree of fierceness level (lol, don't know how to explain that better), Dempseys are a little more mellow (individual basis) but they can fight fairly well, Nics are middle of the road, they can be fairly tough. They grow almost as big as Dempseys. Firemouths also get a couple of inches on convicts, but they are outclassed in fighting by many cichlids. Firemouths can be bullies if the other fish let them get the upper hand. If you have a small aquarium and put in a Firemouth with an established convict, the convict may beat the firemouth to death even if the FM is larger. If you have something bigger than a 55 your choice of commonly availeable tankmates will also increase. Good luck.