on top of the guy at the shop telling him feeding our larger 1000g female 10 mice is unrealistic
You don't have to feed 10 mice, you can but you don't have to and 10 mice would be a lot of food it would be the equivalent of a 200/250 grams rat

I feed are 3 large mice (which is equal to a small rat)

My snakes grow just fine even on smaller prey items, I have my Spider who is 1700+ grams at a year and a half.

Only prior to breeding season would I consider feeding my females more, and in that case it would only be 5/6 mice.