If the water parameters are testing out okay then the tank is cycled. In the 20 years I have been doing tanks I have never had a tank take months to cycle. I just don't get it when I hear people say that it can take almost a year for a tank to be ready for fish. Obviously, you need to add new fish slowly to avoid going into a mini cycle but that is true no matter how old the tank is. I am betting on aggression as well. Are these fish you have added all from the same source? What does the lfs say about it?