No pooping is not a concern, but no urates is.
Make sure you do regular spot cleaning to see that this happens.
I hope someone with a more experienced eye comes along with regards to knowing if you were lied to or if the snake really is that underweight.
I would be feeding at 10-15% body weight or to the widest part of the body (as previously mentioned). Only you can eye that up for yourself, really. But somewhere between 19-28g rat should do. I'd stick to the lower end myself - this would probably be rat pups.