What are the dimensions of the 55g? I had a pair in a 48x20x20" tank for a while and would be reluctant to see them in anything smaller. I also raised a group of six from about 10mm to nearly full size in my old 1400L tank before selling them to a guy with a 6x2x2' 180g, and again I wouldn't feel comfortable having a group in anything less.
As for them being sociable/schooling fish, while that may be true when younger as adults a pair or even single will do just fine. You've done the right thing by starting off with a group if you want a pair, over the years I've kept and raised ~20 uaru, and never have I been able to tell the sex until they pair up and start breeding. Make sure there is driftwood in the tank and feed plenty of high quality green foods, they should grow fairly quickly. I fed mine a mix of NLS, Hikari cichlid bio gold, spirulina flakes and romaine lettuce (and wood!), but they also had a tendency to steal carnivore sticks off my arowana. I always kept the temp warm (28-29C) and the water soft, and be cautious about adding other fish as they can be prone to whitespot (not as prone as say clown loaches, but more sensitive than the average cichlid IME), quarantining is a good idea if you're adding fish from the LFS.
Congrats on buying one of (IMO!) the most interesting and personable cichlids out there, certainly one of my favourite fish!!
As for them being sociable/schooling fish, while that may be true when younger as adults a pair or even single will do just fine. You've done the right thing by starting off with a group if you want a pair, over the years I've kept and raised ~20 uaru, and never have I been able to tell the sex until they pair up and start breeding. Make sure there is driftwood in the tank and feed plenty of high quality green foods, they should grow fairly quickly. I fed mine a mix of NLS, Hikari cichlid bio gold, spirulina flakes and romaine lettuce (and wood!), but they also had a tendency to steal carnivore sticks off my arowana. I always kept the temp warm (28-29C) and the water soft, and be cautious about adding other fish as they can be prone to whitespot (not as prone as say clown loaches, but more sensitive than the average cichlid IME), quarantining is a good idea if you're adding fish from the LFS.
Congrats on buying one of (IMO!) the most interesting and personable cichlids out there, certainly one of my favourite fish!!