I have had a stonefish for over 3 years, Imade a cave just big enough for it to hide in but close enough to the front of the tank so we can see it.
It is about 6 inches long and it never bothers any of the other fish but I do have to feed it with a long stick otherwise all the other fish eat all the food before it gets down to him.
Just make sure his cave is just a little bigger than he is that way no other fish will try to get in it.
My tank is a 125 gallon,with a 14 in. porky, 9 in. stares & stripes puffer, 7 in. niger trigger, 7 inch clown trigger, and a 7 in. volitan lion.