I haven't had experience with guinea pigs babies, but I have had chinchilla babies.

You can get a guinea pig spayed or neutered, but be sure to go to a vet with experience spaying rodents. They are extremely sensitive to Anastasia and if not monitored carefully it could be fatal.

Keep a bowl of mommas pellets available in the cage, they will eat it when they are ready. With bunnies it best to keep babies on an alfalfa pellet, then when they are older they should be switched to a timothy pellet. I believe the same is true for guinea pigs.

Good luck.