old thread, but flowerhorns are not persistent most of the time if the fish they are attacking doesn't die or back down. They do tend to pick on most fish regardless of size, but they will usually coexist without any issues if they don't think they can kill it. I have 2 14" males together with a red devil and they beat each other up, lip lock, then hang out like buddies. The red devil isn't scared to come out and check things out, but due to their size he's scared of the 2 massive behemoths. I don't think either is "more" aggressive.