IF you go with mice, it's best to buy a couple females and a single male and keep them all together from the start. go for younger mice instead of adults. What I do is try and buy the females at one place and the male elsewhere to try and get a diversity to genetics. Some petstores breed their own mice and who knows how many generations they have been inbred.
Rats is the same thing but they can take a little longer to get settled in a new colony and produce offspring, as much as 2-3 months. If your animals can eat pinky rats right away, I would go for a rat colony instead. They are not as smelly as mice since male rats do not possess the musky odor of male mice.