Variety provides them with a wide range of sources for nutrients. Gerbils have a different nutritional make up than rats just as rats have a different nutritional make up than mice. I will be feeding the following to my ferrets: (but I won't be breeding all of them just yet, especially the rabbits)

Rats
Mice
Gerbils
Hamsters
Rabbits
Guinea Pig
Button Quail
Chicken
Superworms
Crickets
Feeder Fish