Milk is a definite no. I believe orphaned babies do well on goats milk - which is the closest you can get to guinea pig milk. Once they're past 3 weeks/the weaning age, they don't need milk. Pregnant and lactating sows do need extra calcium, but this isn't necessarily best achieved by giving milk. If you're looking for sources of calcium, there are lots of veggies which are high in the mineral as well as alfalfa hay and alfalfa-based dry feed.