If they are younger than a week they tend to foster quite well but make sure they have enough energy to compete with the well fed kits. Many fosters will die simply because they are too much smaller and weaker than the well cared for litter. Extra hand feeding, putting them on the doe that is feeding well, or removing the well fed kits for one night may need to be attempted before making them one whole litter. A healthy litter can miss a feeding and survive just fine. Slightly older kits have some risk of being rejected and the difference in activity and health may be even greater causing more difficult but generally up to 2 weeks works fine and some does will accept them older. I just put 2, 1 day olds, and 2, 2week old with my mini rex doe's 4, 10 day olds because she fosters anything and everything. I just flipped her over and put the young ones on her until they were somewhat round and fell asleep before sticking them all in the same nest. This is the main reason I breed that doe. She'll foster about anything. Now my champagnes I have lost nearly every foster because they couldn't compete with the litter that had several good feedings on them.
