Put simply, cross-fostering is adjusting your pup to accept other species as friend, not foe. Other species become part of the pack. You must start socializing them together in the 2-4 month period (this is before the hunting instinct really kicks in). Like in the wolf, this is the time for learning about family, so if they are around ducks, chickens and sheep, etc, they will learn to accept them as one of their own and will NEVER chase them again.
It is best to introduce your pup to all of these animals (mainly the ones they will have access to during their life) as early as possible. Two months is better than four months. As the pup ages, it will become more and more likely to want to chase the other species, especially if they run.
With Terriers, Sight Hounds, and any hunting breeds (including Labradors and Retrievers), if they show any inclination towards chasing a cat or chicken, you must deliver some form of mild correction. In the ideal world, this would come from the animal itself, so if you have a cat that will take a swipe at the pup, for example, then the pup will learn almost instantly not to chase the cat and also that the cat is more powerful and becomes an authority figure (the dog will show the cat respect from now on).
A good alternative to the animal delivering the correction itself is a long lead and check collar. I like to walk the pup around all the other species, and when it chases any of them, I give it a little check. Conversely, when is interacts gently and sociably, I reward with the clicker and a treat.
It is important to limit your socialization not just to animals, and to include as many different things as possible. Something people don’t think about is the importance of introducing your pup to a wide and diverse range of people. I like to include all ethnicities and all age groups, or you will inevitably have some form of people aggression when the dog matures.