If your set on that tank you could rig some shelves to put more rocks/caves on. A grow out tank could also help you. Allowing fish to get up enough weight to defend themselves. There is also the theory of overstocking to help with aggression, but it has it's pro's and con's.
I once saw a hex tank where the guy had build an elaborate system of cross crossing/ descending rock towers. Making the up/down of the tank like a maze. It looked really cool, but also very difficult to manage/maintain. There was no room to get a siphon to the bottom.
I think your best options are getting a long tank, finding a way to make more hiding spots, (maybe you could stack a pillar of rocks going halfway up the tank that they could also swim through) switch out the more aggressive fish, or overstock the tank so there is not room to really fight.