The definition of "pup" varies greatly. A true pup is larger than a fuzzy, but smaller than a weaned rat. They are usually between 2-3 weeks of age. Their eyes will be open and they will be mobile, but they will still be dependent on their mother. The differences between mouse and rat nutrition may be negligible on paper, but most of those who have experience with both mousers and rat eaters will tell you there is some difference in growth rate and body mass. A snake can be perfectly healthy on a mouse only diet, but feeding multiple mice in a feeding can be a real pain and the costs can add up.
The general rule of thumb for feeding young snakes is 10-15% of their body weight every 5-7 days. As adults this can be cut back a bit. Most folks here recommend getting a cheap digital food or postage scale to weigh your snake and prey. This helps new folks get a feel of feeding appropriate sized prey as overfeeding and especially underfeeding is a major problem with new keepers. After a while you'll be able to eyeball it and be pretty accurate.