lI'm not sure where you got that information?Also, buy from people producing rats as feeders, not fancy lines. The difference in litter sizes, growth rates and overall size, are like night and day.
I average 12 to 15 babies a litter and have some of the healthiest rats ever produced from my fancy rats.
They grow as fast or faster and chubbier than any crappy genetic feeder breeder rat.
I have had numerous 17 to 19 count litters also, fancy rats are bred for health and conformity and large litters, after all when you charge $20+ per rat you want as many as you can to sell
You don't need to worry about traits too much but you want the highest quality rats you can find.I don't mean to sound ignorant but do I really need to worry about getting different rat such as dumbos and rexs when I am only breeding them to be feeders?