Cannot really offer much advice as still a "noob" where Tropheus are concerned.
I would agree that a 5' tank is better than a 4', when I first enquired about getting some Tropheus I was advised on this, I now have a 6' tank with two Tropheus groups in there, they all look healthy and are growing well, they show spawning behaviour, I have attempted to remove some of the females to check for holdong and strip but never have much success catching them so leave them in there to avoid stress. They may never hold long term as the tank has minimal rockwork and is quite a busy tank.
I like the idea of adding plastic pipes and removing the rocks, sounds good.
Also just to note, the amopunt of folks advising ti keep Trophs in a 4' tank, ai just wont do it, my own 4' seems quite overstocked , i.e. 2 types of shellies, rock dwellers and a big bunch of young cyps but running 3 external filters...... not a permanent set up...
I have seen Troph tank with no rockwork at all so maybe this might ease the aggression ?
Cheers, Sean