They don't necessarily need to pick on each other. They will get mean towards themselves if they are cramped for room, hiding spots or underfed. Stress will also make them somewhat mean.
I have had naso, salifin, 2 purples and yellow all in together in my 210. They never got in any fights and the yellow and 2 purple would even school around together. The Naso and Sailfin had to move on to larger homes in about a years time. The Naso has a new 850G setup and the Sailfin in a 450. I gave the yellow away to somebody and introduced a BPT, mini hippo and a Mimic. They all get along too with no agression.