My experience with blacks, which may differ from many others, is that they get along in groups of 3-4 just fine till they hit kind of a difficult adolescent stage. I had three together without any issues whatsoever for months. Before I got them they were in a crowd of 7 or more. But I bought three and they were fine, schooling together and loving life. Then one morning I woke up to a ton of nipped fins, and the next morning I woke up to a dead 12" black Aro and another with serious gouging out of his stomach area. I immediately sold one and the last guy left is healing nicely. But obviously something changed in their dynamic and they must've had dominance issues or something.
Babies in a 180 ought to be just fine though. Just keep an eye on them. At the first sign of trouble, split them up. Chances are you'll be OK but just in case you aren't, be prepared to raise them for several months in separate water somehow.