Im thinking maybe a small school of bala sharks and a Geophagus for starters in the 300 gallon.
I don't know. I'm a hardcore 'never get a bala shark' guy. That fish does several km at a shot runs upriver against the current in nature, and there's no way it can get a run like that in even in 300 gallons. They're one of those fish I pity in captivity, and that's no fun.
But Geophagus - yeah! Actually, if I had a 300 gallon, I would get my favourite large Cichlid, Satanoperca daemon. They get to 16-18 inches, and I have seen huge ones. But they aren't expensive and they can be easy to find. The catch is they are more sensitive than Discus, so super light stocking even in a 300.
You could have a big shoal of Hemiodus tetras in the 'small' tank, with 50 Corys and whatever else appeals, if you go South American. West African could be cool too. Or real (not Satanoperca) Geos, or Guianacara!
If I lived near the stores Juice is close to, the world would be possible. The Wet Spot has my ideal fish I can't get...I'd have a pair of Pelvicachromis sacrimontis black, whatever Benitochromis they get in, lots of African tetras, a la
@Magnum Man ....