Years ago, I used to raise guinea pigs for a pet store. I'd have up to 80 babies before Christmas. I've had several litters of 9 pups (all surviving). Several of 7 pups, too. A mom will feed any baby. So, fostering some of them will work if the litters are really close in age or if you can wean the other litter at 3-4 weeks. I also, used 1 part V-8 juice to 1 part milk in a gravity fed water bottle as a supplement. I decreased the milk portion as they got older. This was an old-time formula. I would leave the bottle in the cage for an hour or less, so the milk did not go bad. I would do this several times a day because they loved it.
Make sure you give the Mom a rest after this litter. Large litters generally happen when a female is mated right after giving birth.
Dale-Ann