Starting off your snake hatchlings with Day Old Pinkie Mice, working your way up to the large and jumbo sizes as the snake grows is standard practice for many Colubrid species. Some species such as Royal Pythons can be a little more selective in what they eat as hatchlings, if they refuse Rats initially, Mice or Multimammate are a great alternative.
Day Old Pinkie Mice are high in fat, making them ideal for young reptiles with a lot of growing to do. Adult mice are generally healthier than young rats of a similar size due to the ratio of muscle to fat, making these a nutritious food source for your reptiles. Although adult Corn Snakes for example will happily eat a Weaner Rat, it is much higher in fat than a Jumbo Mouse, which over time can cause problems with fatty liver and other obesity related issues.