An aro is probably not the best choice for a tank this size. Especially with the large messy predators you already want to keep. And a single bala will be living full of stress around all of these predators. These sharks do better in numbers of 3 and above (shoaling species).
With these large predators/messy eaters on your wish list, I'd recommend going to wet/dry filtration simply for the large bio-filter capability. The pump needs to be able to handle 950 to 1200 GPH at a 4ft head to keep ahead of these heavy waste-makers. This setup will allow you to only have to do a water change/gravel wash once a week instead of the 2-3 times a week your current setup will require with this bio-load.