You should always keep rats of the same sex together in a cage. If the opposite
sexes come together, you will soon get a litter of up to 18 babies - and the female
can get pregnant again almost immediately after giving birth! So you should never
keep the father together with the mother, especially near, during, and after she
gives birth. Your new babies are capable of having babies of their own as soon
as five weeks after their birth; so you should seperate them into same-sex groups
soon after they've fully weaned (stopped drinking momma's milk and gone completely
to eating solid food).
If you do decide to have a rat family, PLEASE be sure you will be able to easily
find homes for all of the babies. There's no worse thing than an owner allowing their
rat to have multiple litters and having nowhere for the babies to go. This can result
in some unfortunate endings for much of the litter: being cut loose outdoors, being
flushed down a toilet, or becoming food for the neighbor's snake.
Get ahold of the people who raised the rats you later mated. If they're a reputable
breeder or rattery, they can give you addtional tips on this subject.
Reference material on breeding pet rats is also available at your local library.