Dummy eggs can be added as needed. When there is only one baby, it may not be cared for like a nest with two or more chicks. If there are other chicks the similar in age in another nest, the single baby can be added to that nest. Sometimes supplemental handfeeding may be necessary. Sometimes it is necessary to mix species' eggs in the same nest due to emergencies, but this can lead to problems.
Some species grow more rapidly than others or beg louder.
These larger and louder birds are often fed more, causing neglect to the smaller, quieter species. Societies will also favor their own babies over other species, therefore, you should avoid mixing species whenever possible. Not all Societies are good parents of other species or even their own. Some Societies won't feed babies any food except soaked seed or eggfood, others only dry seed. Once you have a pair of societies that will raise a different or rare species, I highly recommend only letting them raise the fostered species, never letting them raise their own.