Hi you will find cockatoos are fine in your community I used to keep them and bred a few.. The first few bunchs were in a well stocked danios, neon tetras, siamese fighters, Pearl Gouramis firemouths a few catfish etc etc..) community small (60 x 30 x 30cm) tank, and they never hurt a thing..
Im actually get a pair of cockatoo's (orange flash) from a wellington breeder who is driving upto Auckland tomorrow to drop off fish.... He is dropping off 100 odd fish to a few stores tomorrow the two I know of are Jansens in Botony downs and on the north shore (but he may be going to more stores I dont know about) these fish are getting pretty hard to find now when I first had them I was in Palmerston north and could get them relatively easlily and cheaply but that was a while ago... If your really keen to get some and dont have a place to get them from one of the jansens stores in the next few days may be a good start..