An acclimation box where everyone can see each other is worth a shot. But have you confirmed that aggression is the issue? Are you quarantining your new fish? Even if it's not to treat them for disease, I have yet to see a fish in a fish store that couldn't stand to gain some weight, many are downright emaciated by the time you get them home. I use the QT period as much to get them eating and confident as I do to treat them for disease. Dropping a fish that's half starved and has just been flown half way around the world in a dark, tiny bag in to a tank full of healthy, aggressive resident fish is a rough way to introduce any new arrivals. If you've got low levels of ecto-parasites in your system like ich that your resident fish have all developed acquired immunity too, the combination of stresses would be enough to take out most fish.
If you're not already doing it, I'd recommend bringing your new fish home to a QT tank where it's just them and you can introduce all the foods they'll need to compete for in the big tank. Once they've visibly gained some weight, then I'd introduce them in an acclimation box and wait until your aggressive fish stop taking runs at it. My powder blue would have killed my copper band had the copper band not been in perfect shape when I put it in, but because of the QT process the copper band was able to defend itself AND compete for food until the PBT lost interest, which took about 5 days.
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