what are they competing for? is the tank too small? how often do you feed them? do you have things in the tank for them to hide in or swim around?
My big peacock bass gets testy when he's hungry, and i know that if they don't have enough swimming room they will pick on each other, they will also pick on each other just for fun..
having decorations and plants gives them things to hide behind and swim around which helps to calm them. i have tall fake plants and a huge castle in my big tank (150 gal).
And yes, you can keep an arowana with them.. at least i do.. i have a beautiful 12 inch long silver arowana that is in my big tank with my big pbass, the snooks and my TSN. my pbass and the arowana are best buddies, they are nearly inseparable.. they get in little tiffs now and then, but they never hurt each other.