The mice IMO are more dangerous than rats. An adult mouse can be very defensive/aggressive but a small weanling rat the same size is still rather helpless and naive. I would feel much safer leaving that smaller rat in a tub with a snake than a mouse of the same size and I feed 100 snakes a week, mostly live.

I would start with live small rats and then when you feel they are eating them without hesitation then try feeding pre-killed.