This is very true.. In the case of rats, no more than 1 or 2 should ever be put with a nursing ASF in an emergency situation or you'll endanger the lives of all her babies as well. They can handle baby mice easily tho.
I had an older ASF colony that wasn't producing much and when they did, babies would disappear. I eventually figured one of them must be killing babies for some reason. I allowed the pregnant female to have her last litter and gave them all to a different colony to raise before cycling out that old colony.
ASfs are great fosters.