A 55 gallon in the end will only fit two pairs, possibly even only one.
But experience will let you know how to handle that part. Starting off with 6 juveniles is a good way to do it so you can ensure you do get a pair or two out of them. Their behaviour with each other and aggression toward each other will let you know when its time to do some removing of fihs.
Cichlids are territorial and I think you made the right choice going with a species tank - mixing single cichlids is popular, but it does nothing to bring out the best of the fish and creates a higher risk environment.
If you were not interested in actually breeding them, and went with just one pair, then you can actually add peaceful-but-tough non-cichlids with them. The trick is to have nocturnal bottom dwellers which will eat their eggs so there will never be breeding aggression. I kept a pair of Nicaraguensis like this for many years in a 75 gallon with a huge school of serpae tetras and 4 synodontis multis
cheers