either get more caves or get more fish... really no other way around it if the aggressors have claimed the whole tank its not going to stop... when you either get the rocks or the new fish, take all the fish out of the tank, put in the rocks (if applicable) and put in the least aggressive fish first... let them settle for a little while, then add the dominant ones... the rocks will throw them off thinking they are in a new territory, if there are rocks they will claim a smaller area then just the whole tank...
If you want to go open tank... I suggest waiting until you are fully stocked, then pull the rocks out... by then the tank will have enough fish in it the more timid fish get lost in the crowd, and dont get as stressed