Julidochromis aka Julies are very cool fish and will do better if you provide some rock piles.
Unfortunately Julies won't get along very well with most of your current stock. Julies require hard water w/ elevated PH, while your South American cichlids do better w/ nuetral to acidic water. Although the Convict does fine in hard water, its aggression level might be a little extreme for the Julies.
A baby Blue or Channel catfish grows very quickly and will be able to eat the Julies as well as everything else in the tank, in short order. Its best to keep African Rift Cichlids w/ Africans & New World Cichlids, such as your Severums & Convicts, w/ New Worlds. Hope this helps, good luck with your tank.
i threw my last 2 julies into my 240 gallon tank just to get them away from the brichardi colony that had overwhelmed them and decimated their numbers and didnt expect much to happen. the 240 had convicts, texas cichlids, firemouths, and an oscar. now i've got about 2 dozen julie in there and theyve eaten all the convict fry for the last 6 months