I agree with them 100%, which is why I said to minimize your fish load with competition with the space they swim in. We designed our 75 so he would be the only free roaming fish... everything else was rockwork fish or two clowns that hover.. he had the rest of the tank for himself.
If you buy a 3" I doubt he will outgrow it in one year.. but it's possible with a healthy fish and a lot of food. With a lot of fish in the tank, you will have territorial issues for sure.. bc as they get bigger.. they get more aggressive.
IMO if you don't want to upgrade your 90, stay away from them. The Achilles will be even worse. I'm not trying to be TP, it's just how it is with these fish. Even in my 120, it's too small for a full grown PBT.