Yes I think Pacus should be okay(should because I have never owned a pond)...As for oscars and RTC...eventually, a grown RTC can eat a fully grown oscar...
What would be the foot print of this 1800gal tank? A 120'x1'x2' tank is still 1800gals.
As for RTC's and oscar's... Regardless if you grow the oscars to 12'', a 2'' RTC will catch up to the oscars in 6 months or less. Expect the RTC to grow to 2' in the first year. Pacu's I have no Idea about there growth rate. But, I'm sure it's fast to.