I think people have grow out tanks because putting tiny fish straight into huge tanks, especially a single fish as you are planning, will mean your huge tank will just look so empty for such a long time before the fish eventually matures. Nothing wrong with doing it that way, in fact some would say that is better than keep moving it on from grow out tank to grow out tank until it eventually reaches a size where it can be transferred into its final home.
But occasionally unforseen circumstances can arise in life and the end plans for a huge tank for your growing out fish might not materialise, leaving you with a brute of a fish in a smaller tank than you had planned.
So the fact that you already have your 500g is good, it will just be a fair while before that little brute looks the part in such a big tank.