Although those extra gallons in the sump will help maintain water quality, the tank is basically, only a 150 gals of living space, which has only has enough territory (footprint), for maybe 2 adult oscars, and a few clowns.
I agree with the others, if you want the oscars, forget the other cichlids.
On that subject, what are the parameters of your tap water?
Soft with low to neutral pH? Good for adult Amazonian species like oscars, Heros, Uaru, some Geophagines, and Hypseacara.
Hard (250 mgL or above) with high pH (7.5 or above)
Good for cichlids from Central America like JDs.
The problem with hard water and oscars (and the others mentioned above), is it tends to produce chronic maladies as they age like HLLE, so just when they reach the optimum adult size, which may be your goal, they end up scarred with hole in the head.
And hard water in tandem with elevated nitrate seems to exacerbate those conditions.
So if the tank is even slightly overcrowded, daily, or every other day large water changes will be needed to combat the ill effects of the hard water nitrate soup.