I have at the moment 40+ cockatoo's in a 50ltr tank without any issues, they even eat from my fingers.
I also have had another 50ltr tank with 2 pairs of rams, 2 panduro, 2 other apisto's (possibly borellii) and a pair of cockatoo's and a few misc tetra's, guppies and bristlenoses with no problems at all. Rams even bred in there.
It is good advice to give them some of there own space and I have seen them get a bit protective of there fry but very seldom with cockatoo's get agressive beyond a bit of chasing. (unlike agassizi which are just plain mean when they have fry)
For dwarf's is not the volume that matter as much as the size of the foot print because they will hang around middle to bottom most of the time. Dither fish like guppies or danios that hang around the top will help alot to bring them out for food.