The problem is that from one person to another a certain name may not be the same thing and not all feeders are the same size either based on who the supplier is
Rat pups are generally 2 weeks old with eyes open.
At a month old BP are generally fed fuzzy (on the larger side) either way just eyeball it so the prey is the size of the girth of your BP.
Right now before trying to switch you need to get him to eat for you, that will be your first job as a new owner, once he eats for you with consistency (3 to 5 meals) you can consider to switch him.