I think the more fish the more the aggression is spread out amongst inhabitants.
Considering the fish you have stocked together i reckon you are doing very well! Definitely keep off the feeders as i have trouble introducing new fish unless they are bigger then one of my dominant pbass.
I am going to try some plastic plants but so far my tank is doing great with little aggression - i have 4 x cichla monos, 4 x silver dollars, jardini, 2 x oscars, sailfin and few others.
Keeping the fish well fed also reduces aggression in my experience.
Great thread lets keep it going!