Is this a good stocking for a 300 gallon?

why do people always get big fish when they have a big tank?

Why don't people realize that they have a VAST variety of the most beautiful fish that aren't huge. You could turn a 300 into a beautiful heavily planted tank with the most diverse gorgeous array of fish.

Just my opinion.

And there's not enough space for all of them. Bala sharks get huge. Oscars get huge. Stingray get huge and are demanding. The pleco will get huge. Just too huge. The arowana especially as well. I'd cut down to one or two stringray, the arowana, 1-2 oscars, and I would add a small school of silver dollars.

No. Too small for all of 'em. For one, the arowana alone requirese 300 gallons.

I've answered your question before I think.

note: I do not agree with having stingrays, and believe it's cruel to keep them. But that's just me I guess.

list sounds ok since you removed the more aggressive fish.

Add "dither" fish, either giant danios or some other relatively cheap and hardy fish. Maybe some barbs, like tinfoils or large tetras like silver dollars.