Buy your rats as weanlings (they grow fast). Females should be sisters. Put 1.2 - 1.4 in each tub depending on tub size. Put your hands in the tubs ALOT. Give them treats. This will earn their trust. I prefer dumbo rats, they are cuter and seem more docile than regular rats. You should not have to worry about separating anyone this way. Also never let them run out of food or water and keep the bedding clean. You will get more babies if you and the rats are all friends.