Blue Dolphin cichlids get to 10 inches when fully grown - but it takes them a while to get there. I've had mine for 8 months now, and he's still only 6 inches long. Whilst they have a reputation for being fairly docile, the juveniles are quite territorial, and will try to chase off anything that's near their bit of tank. A typical territory for mine will be around 2 sq ft. Fortunatley, they have small mouths and are incapable of serious harm to each other, or pretty much about any other fish - but they will lip-lock, so watch for damage there.
In a 55 - probably two quite easily, that would give them enough room as juves to settle down, and once bigger they should be easier to handle in general.
I've got mine in a mixed tank and he loves pushing the others around (except the Oscar). Feed on prawns and flakes, tough little fish with regard to disease.
Mine has stripes on his sides - just come from the LFS where they have one with two large spots, so marking differs. Lots of character, interesting to watch.