Because of the huge adult size of the Silver Arowana, and it being an aggressive predator, there are very few common aquarium species that can be kept with this fish. Tankmates have to be large and robust. In the wild, this fish survives by jumping for its food which includes insects and small birds as a result of this, the tank must be kept covered at all times and even weighted down as the fish grows.
What are the exact measurments (length, width and height) of your tank

?
Royal plecos are very territorial towards each other and often towards other large plecos too, they also don't grow as large as commons.
The commons may become a little territorial towards each other, but this shouldn't be a problem in a 518gallon tank with a good decor arrangement.