Re: Question about feeding.
If the snake is above 300-350 grams, he/she could handle a small rat. Just be sure to monitor the feeding. Don't just drop the rat in and leave it unattended with your snake. If you don't have a pair of feeding tongs, get a pencil or something long that you can use just in case your snakes first strike is not a good one (use the tongs/Pencil to keep the rat from biting the snake). Most likely if it won't do any real damage to the snake's skin, but you wan't to be safe than sorry. Only leave the rat in with him/her for 20-30 minutes. If it is not eaten in that time, take it out. Enjoy your new addition and welcome!
Re: Question about feeding.
One thing that you can do to increase your chances of a successful feeding is to pre-scent. Most pet shops have some type of little transport boxes for rodents. When you get the rat home, set the box on top of, or next to the enclosure. This will allow your BP to smell the rat first, and become alert and get into feed mode. Sometimes the snake will just poke it's head out of the hide and look around, and sometimes they will come all the way out and search for the prey. Once you see that your snake is interested, put the rat in the enclosure.
Since you will be feeding live rats from a pet shop, it's a good idea to ask if they have a return policy should your snake decide not to eat. If they don't, then I would suggest keeping a Critter Keeper with a water bottle on hand, that way, if your snake doesn't feed, you have a way to house, feed and water the rat for a few days until the next feeding attempt.