So avoiding the whole tank size thing. Would a mimic, hippo, and powder brown get along? Plenty of hiding etc etc. The mimic and hippo already have been together for over a yr adding a powder brown first and than the other two?
I don't think you should ignore tank size, as a bigger tank will help reduce aggression. I think its the length that matters more than the volume (within reason of course)
The tang experts will have to chime in though on addition order.
TANG POLICE!!! Just kidding!! I've always taken the approach of see how it goes with my tank... It's bit me in the arse a few times, but for the most part I've been very pleased with my unorthodox fish selections!! And the fish all seem happy as well!!