If Skittles can eat 3 pinkies...should he move on to bigger food?

[shelliebear]

I feed my one year old corn snake Skittles 3 pinkies a week, as he's too big for one, but now 3 barely stretch him out at all, as in, you can't really tell he's eaten after he's eaten.
If he can eat 3, should he move on to older pinkies? I found a mouse breeder who will sell me ~6 day old pinkies for $1.33 apiece or 15 of them for $20.
First off, is this a good deal, as my insight tells me no (that's around petco prices....which are bad) and second off, will the pinkies be too big, do you think?
I just don't want to over feed but the guy selling the mice did say if he can eat 3 pinkies he should be on bigger food.
Thanks.