Is this a good stocking for a 300 gallon tank?

I think you'll have problems buying those fish in the sizes you need.

an oscar will grow fast enough to eat the balas, and the loaches.

the stingrays will vacuum up any fish they encounter that it can fit in their mouths, and even things they can't fit in their mouth. They'll do it at night usually..

the stocking is ok, but for various reasons I usually advise people to be cautious about choosing stingrays, and arowanas. They are sensitive fish that don't do well in tanks for very long. You do have more space than most people get for them, but it's still not enough for them as adult fish.

your filtration has to be incredibly good. I'd suggest a 50 gallon sump based system,,,,, at least..!!

personal note: I hate parrots, they're a genetic freaks, Frankenstein fish, barely able to live because of their screwed up intestines and deformed mouths.

but, it will be an impressive tank with the stocking you have here.

It's ok for now. Might get a little more crowded once they all grow up some. But they'll probably knock the numbers down a little for you. The clown loaches, and bala sharks will be the first to go if there's a problem. They aren't really aggressive fish. Very nice collection btw.