Although I agree that tiger barbs are the best choice if anything is going to work out, you may not even be able to keep them. I have a 55 with a large severum, a convict, and a firemouth as the larger aggressive fish, and they kill every dither I try to keep, including TBs. I've since given up trying to keep dithers in that tank.
Have you tried rearranging the decor? That might throw them off a bit when they have to re-establish territories, and sometimes can help with aggression. Or, can you add more hiding spots in your tank, like caves, rocks, plants (probably would have to be fake ones), or something else that would help to break up the line of sight from one end to the other? That might help too.
