Believe it or not I qt nothing. I have a hospital tank and fish trap if needed but treat nothing prior to intro. My display tanks all three have ich so my fish have developed immunities over the years.
Occasionally the passer will bully the emperors (rarely) even though the emperor is the largest (6") the queen is about 4.5" and the passer is about 4.5".
Currently my emperor randomly passed but I replaced him before the fish could really get used to his absence. Same size though, no aggression. My queen is surprisingly passive and has been since the beginning. He and my naso died during a tank move (more succinctly a couple days after)
If breaking "rules" in this hobby, it's best to really break them. For instance, two large Angels would have more chance of issues than three. Same with tangs. My other 180 has achilles, PBT, yellow, purple and desjardi sailfin. No aggression issues for the most part. Achilles most docile fish of group. Always add tangs in twos or threes. If not, aggression kills are possible. My purple is the tank boss aggressor.