Itd hard to answer because it depends on what type of aggression and also cpuld have many variables, what happens in one tank can be totally different than what takes place in another.
My ornate was a jerk to smaller polys no matter if there were hides or not. But never messed with large polys.
Had a small pbb that was a butt hole with no hides at all.
The charlie manson endli chilled out when i removed the dw but eventually went back to being an ass.
Aggression is kind od complicated atleast ime. Sometimes is just a dominance thing where the aggressor nips at others to let them know whos in charge and isnt too serious. Right now i have an sp.tinkisso/dabola that took over as top dog and will occasionally nip, i feel that if i put a hide in or just a large dw she'll claim it and the aggression would get worse.
How long has your current group been together ?
The current poly group that consists of 3 ornates and 3 sens has been together maybe 2-3 months now. Currently the worst aggressors in my tank are a small red terror and a small green texas cichlid but they are only aggressive to other cichlids especially oscars which they really seem to hate. but the muliple low level hides and five silver dollar dithers and other cichlids help to spread and reduce the aggression. Yes I am overstocked to growout and reduce aggression. I will have to take some pics of my current setup when I get a chance. The Poly's haven't shown much aggression to each other or anything except the ornate which i mentioned and a larger sen I purchased more recently than the other two sens. He decided he wanted to eat my spiny eel a few weeks ago when I thought he would be ok in that tank and ignored by the polys. Turned out he was not large enough. I rescued him as I saw him being swallowed by the larger senegal but he sustained too much damage during the incident and could not recover
Most of my aggression issues are cichlid related and not poly related at this time I am guessing they have just been establishing a pecking order. My tank is in my living room but away from opening doors and much traffic so the poly's don't really get freaked out. I always move slow when near the tank as well to prevent freaking out the gars.