Guinea pigs have babies quite often. Gestation in guinea pigs is somewhere between 63-70 days long. If there is a male in with her when she gives birth, then right after the last one is born.
No. I know because they might try to mate with eachother, if they are different genders if they are the same gender, they will fight, so put them in different cages!
When they are first born and up to 4 weeks old the mother will feed them milk that she makes for them. She will teach them by showing them what to eat and drink. and how to be with other guinea pigs.