Since this is your first time with cichlids.get down to the basics.90% of the species is aggressive and is not for a begginner.Convicts are a good choice to start out with,but be weary.They will breed like rabbits and kill fish 2 times there own size during mating.The male convict is bigger than the female and normally has a bump on his forhead.The females will (always) have a orange/redish belly for mating purposes.The females are always smaller than the males.
Convicts and dempseys will be fine.But introduce both in your 50gal as juvenilles so they can grow into the tank as mates.No more than 2 dempseys in your tank,and no more than 3 convicts due to territory and mating.Because trust me!Your convicts (will) mate!
The same rule is pretty much with the dempsey also.The males are bigger with a lump/bump on there forehead.
convicts- 3-5"
Jack Dempseys- 6-9"