Would a cocker spaniel be okay in the same house as a rabbit and a cockateil ???? ?

I would't do that. Cocker spaniels were originally bred for hunting in England in the 1800s.

Working? What does he work at? If he's a therapy dog, it should be easier than if he's a birder. I grew up with a spaniel, a rabbit and a gerbil all living together in the same house and the dog would 'go find' the gerbil (when he would escape from his cage) and gently carry him back to us so it's possible but don't forget that dogs are predators and it is you who must create the harmony and maintain the peace by being a leader to your dog, making sure that he gets plenty of exercise and behaves respectfully toward all of your friends, including the furry and feathered ones.

Have fun.

Cat`

I think the working gun dog would enjoy the bird and rabbit very much.

Your bird and rabbit may not feel the same.