I currently have 5 Black Aros in my 240. I know it's too smal...working on getting a 700 gallon before the end of the year..
First, I have 3 of them in there, they grew from about 12" to about 14" in a month.
Out of the 3, one of them was getting beat up constantly, but he hold his own ground just fine, other than a few torn fins.
During this time, I leave light on during night time and leave a blue moonlight at night.
I kept them well fed every day.
After a week, all 3 of them were getting along...just fine...I was planning on putting my Lei in the big tank, but I was afraid with 4, the aggresion will start again especially from the Lei..The lei is only 8"
So I added 2 more a month later...these two were pretty small, about 11" or so..
First couple of days, the smaller ones were being bullied..then it stops. A week later, the smaller of the 2 smaller ones was beat up pretty bad...So I separate him as of last night...
But, the other smaller one is getting along with the 3 larger ones..
In conclusion, Fish can sense weakness, they will bully the weakest one of the pack.
Keep them stress free by keeping the water clean, well fed and have a enough get away space. My tank is nicely landscape with tall fake plants and large driftwood.
Keeping PH somewhat within the 6 ish range helps to calm them down as well..
As long as your fish can hold his ground and you have a back up plan, I don't see why you can't have a community tank..
stan