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.