I had my FRT at 5 inches and my aro at 9 inches and everything is fine. I have a Indo Tiger, Aro, FRT, and Prochilodus all in a 180 gallon tank. My son calls it a Kung Fu Panda tank.
If you do water changes every two weeks, you will be fine.
i have my frt turtle with my aro w/ no problem. Frt at times are slow & will bite bottom dweller fishes like sting rays or birchirs.. aros swim more freely 24/7 on the top most of the time.