Alot really does depend on what size tank you have them in.
A pair can be kept in a 75 gallon tank, 4 foot by 18 inches by 20 inches. But it woul not be a good idea to have any other cichlids in with them.
It probably isn't possible to sex them at this young age, but if you end up with two mated pairs, it would be best if they were in a 6 foot tank.
As for tank mates. Well let's stick to other South American fish.
High bodied tetras would be good as some schooling fish, anything like Lemon tetras, Black & Red Phantom tetras, Black Widow tetras, Bleading Heart tetras. Possibly Emporer tetras, but you want to stay away from the slimmer, bullet shapped tetras. There are also some nice spotted silver hatchetfish going around at the moment.
Then you can look at some catfish for the bottom of the tank. There are many corydoras catfish that grow in the 3cm to 6cm range available at most LFS here in Australia. They do best in groups of 6 or more, and are good at scavenging uneaten food from the bottom of the tank. Another favourite are bristlenose catfish, they eat algea found in the tank and grow to between 10cm and 14cm.