5 or more is your best chance, for example my 550 gal is a comm tank and I currently have a RTG, Green, 2 x Silver, and a black ( no fighting). I also have a 280 gal with smaller aro's, there is a Super Red and 4 silvers ( about 10", no fighting ). I have found that if you raise them together they take the comm situation better, I have tied putting together bigger one's that have never been with other aro's before and it didn't work well. Australlian Aro's do not work in comm tanks at all. I'm going to put all mine together in a few months and remove one big silver that has crooked back. the only problem with comm tanks is having a tank big enough, I would think that in a couple years that I'll be looking to build at least a 1500 gall tank, and the upkeep for good water conditions. Nitrate is the hardest to keep under controll, recently I have added 3 small plants in an attempt to help reduce nitrate and it has been working very well so far ( before plants = 50 ml/ltr, after < 20 ml/ltr).
Good Luck.