Well, I have four turtles. 2 adult red-eared sliders (10in & 8in) and 2 map turtles. I don't have an arowana, BUT all my four turtles are afraid of my 4-5in goldfish, and my old pleco, from the time he was little, would open up his fins and terrify the turtles (funny as h311). With my turtles I'm not worried for my future plan of a 300-400gal w/ turtles & arowana. REALLY good filtration will be needed! If your not willing to risk it, then don't do it ! Good luck!
It depends, I have 5' frt with my rays and aro and no problem so far. I also have 10' frt that is super friendly and will keep them together when I get bigger tank.