They only way I would be concerned about what the rats/mice were eating is if they were being fed rat poison or were living in the wild. Dog food is fine for rats, although it doesn't always have everything mice/rats need to live long healthy lives, but that doesn't effect your snake. They also don't need to eat name brand or off brand, it really doesn't matter as long as they can live off of it. Cheap dog food has more grain, expensive dog food has more protein, so theoretically the cheap stuff is closer to the kind of food they should be eating.