I was told that at most farms they have different varieties in each pond, due to the fact that pairing of the adult fish is not very easy. So basically they have more fish to give "more chance" for spawning to occur.
Not too sure about the validity of this, cause it would make more sense to have single variety ponds in order to keep purer strains.
At any rate, with red arowanas you can't really tell what type they are when they are really young. Once they get big enough to show some scale and fin color they are then graded and sold.