Thanks for all the help guys!
I went out and bought another tall plant yesterday, and it seems to have helped quite a bit. The aggression has toned down a lot, and although there is still the occassional flaring match I haven't seen any nipping or chasing since yesterday.
I'm still treating the water with Melafix to prevent infection as two of the bettas (Storme and Thorn) have nips out of their fins. I nearly considered removing them because they just wouldn't quit fighting eachother, but luckily things have settled down and are looking pretty peaceful at the moment. No other bettas seemed to be particularly targeted by aggression, but I'm going to be monitoring the situation very closely for the next few weeks.
Besides a few torn fins all of the bettas seem happy and healthy, they are eating normally and their colouring is already improving. I'll keep you all updated and let you know if anything takes a turn for the worse, your advice is always welcome! :)
Sophie and Luna: http://www.bettafish.com/attachment....1&d=1386024346