Can you have other cockatiels in the same cage as a breeding pair of cockateils?

You can, but it really depends on the personality of the breeding pair. Sometimes, the breeding pair will be so territorial that they will not breed with other cockatiels in close proximity, and will toss the eggs out of the nesting box. Others have no problems at all, and several pairs of cockatiels will be breeding at the same time. It is important to make sure the aviary is large enough to accommodate more birds, and that each of the breeding pairs has sufficianet privacy, for example, placing nesting boxes at opposite ends of the cage.