With that age difference it's likely the older kits would crowd the foster out. What happened to singleton? If you need to use the mother of the 11 day old babies (eg if the mother of the singleton died) I'd recommend holding the dam in milk down twice a day for the singleton to nurse, but keeping it in a separate cage/nest from the dam and her own litter. You might have to give it supplemental heat like a heating pad if it's alone without siblings to keep it warm.
The mother had 4 babies but 3 were stone cold and the other one was really cold but just alive. She built no nest or anything pulled a little fur but that's it on the concrete. Not in the nest box. I dont know if go put the one on it's own with the mum or surrogate because it's on it's own and I dont know if itll survive on it's own.