Strickly speaking you are all right
However, i also believe that if you feed the arowana feeders, you dont have a chance but if your arowana has never been fed feeders and they grow up together from very small, it can work
I have a just under 80cm silver aro with an electric blue acara and two baby Uarus..
I have done this numerus times over the years and she has never eaten a small fish
Flash my Red Arowana, is now just under 50cm, she has two electric blue acaras that have lived with her since she arrived as a baby, she has never eaten a smaller fish that i have added to the tank either
So.. it also depends on individual fish, circumstances and feeding habits.