I’ve kept kookaburra with a variety of slightly smaller birds without problems. The problem comes when/if you try to breed those species with kooks present. Then you need to worry about eggs and chicks. There are a lot of aviaries I’ve seen or worked that are great exhibits, but just don’t work for breeding
Was the flight large enough and was there enough cover for the smaller birds to hide from the kookaburra?
I've always thought, in theory, it would be nice to see kookaburras in a mixed species flight with larger species like cockatoos and other larger bodied parrots, straw-necked ibises, waterfowl, and maybe masked lapwings but after reading your comment, knowing what I've read, and the pictures I have seen, maybe it is best to have kookaburras in their own flight.