I keep and breed
A. Cacatouides and here are my thoughts after research & experience:
1. I would not recommend keeping any sort of bottom-dweller fish besides more apistos (we'll get to that) and perhaps a bristlenose pleco (my breeding pair are not bothered by the bristlenose at all). I have tossed some shrimp in before without any issues but I can't say that they wouldn't be attacked at some point (especially if they shrimp have babies)
2. Cockatoo's are not very aggressive beyond what is needed to protect their territory, when given proper space and decorations.. They can get stressed if you place them in a tank with fast moving, even moderately aggressive fish that are likely to invade their territory (like some tetras). I had some zebra danios in with my breeders before and they didn't mind each other - I think they actually encouraged the cockatoo's to come out more because they felt that it was "safe". If I had to recommend something more beautiful than the typical danio, I think that you'd find success with Cardinal Tetra's as their water/temp requirements are very similar.
3. If you are attempting to breed them, though they are protective of their spawn, dither fish (such as Cardinal's) can and will eat some of the fry. If you are wanting a high fry survival rate, I would recommend not having dither fish. An established pair will likely not need them.
4. Harem breeding. One of the most interesting things about cockatoo's in my opinion is that they are harem breeders. While you can definitely keep a singular pair with success (which is what I do), in such a large tank you could very easily have one large, dominant male and 4-5 females. A general rule of thumb is at least 1 apisto cave per female.
5. Decorations: Cockatoo's do not like large caves, or caves with large openings. They do not feel secure. In my opinion, they are a very skittish fish (especially the males). Apisto caves (like
this ) are perfect as the female
just fits in there (the male cannot access it). The male will benefit from plenty of driftwood, plants and general cave formations but will likely not utilize larger caves as he's always "patrolling" the perimeter of his territory.
Have you already found a supplier of cockatoos? If you are interested, I have some that are nearing the 1 inch mark for sale (located in IL, can ship).