In the meantime, I dug around and found this clip I took a few years ago when a single, still nearly hairless and pink, baby opossum boy arrived.
I could not get him to stay sufficiently warm and comfortable with just a heating pad, so I decided to stick him in with a litter of 3 week-old baby squirrels. It was his only chance at survival. He snuggled right in with his new litter mates and from that point on he ate well and thrived.
When it was time to move the squirrels downstairs into my rehab room and into the cage you see in the video, i felt it was time to separate the little opossum from his a bit unorthodox nest mates. But he started to run around in circles and cried out for his litter mates and would not stop crying. He also would not eat. I realized that he was too young to be alone and put him back with "his" squirrels. He was elated and they seemed to like seeing him back as well.
A few weeks later, when the squirrels were 12 weeks old, I moved them outside into their pre-release cage, and at this point the opossum was also old enough to manage by himself. This time he didn't cry non-stop when he found himself alone, and eventually he, too, was released back into the wild.
In nature squirrels and opossums usually do not meet unless an opossum climbs a tree and tries to get at a nest of newborns. I doubt that by the time I released him he even remembered his litter mates. He was a pretty little guy, though, and very sweet.