Cavies are efficient but Im not sure they are a lot more efficient then other small meat type animals. They can reproduce at 3 weeks but its rare and for females dangerous as they are not big enough yet to have the size to carry and deliver babies. Males surprisingly don't often reach maturity until later, I have had plenty of males that don't breed until after 4months of age despite being caged with girls earlier. Sometimes I wish they would breed quicker though. They have small litters when compared to bunnies, 1-3 is most common with 4-6 being somewhat rare. They dont reach full size until 5-8 months of age and as far as eating they are "pigs." I feed a lot more then I ever did with my rabbits. Rabbits however can also be highly destructive, can run fast and jump, they have a pretty pungent urine odor and fight when maturity hits none of which is really suited for living in large colonies on floors of a house.
As far as moms and babies cavy milk is important and baby guinea pigs would die without their mom, though they are born with eyes open ready to go and they do nibble food starting pretty early they are fairly helpless in the first couple weeks and nurse almost constantly. If something does happen to mom in a colony with other nursing moms they would easily be accepted by another nursing pig even male guinea pigs will clean and cuddle babies.
I have raised and shown g-pigs for the last 20 years and right now have around 60-75 pigs at anytime just depending. At this point though im not considering any endeavors into the guinea pig meat industry....
