In general, having more aros is supposed to disperse and thus reduce aggression in aro comm tanks. However, there are some aros that will be bast*rds no matter what the situation, so it's just basically down to the individual personality of the fish.
Silvers and blacks are supposed (in general once again) to be more placid than the other species, but their problem is their max potential size and the size of tank/pond required to house them for life.
How big is your tank?