I realize it's better to buy the feeders in advance so they can be fed and hydrated better than they were in the store.

Is there a certain amount of time I should buy the rat in advance? Also, I've heard single rats don't do so well, but I'd really rather not be keeping rats all the time. Not to mention, I don't want them to get too big. (I was told my snake eats mediums).

What I had in mind was maybe buying the rat on Wednesday for a Saturday feeding.