I would only take the pup if you are 100% sure that your sheltie will get along with it.
It wouldn't be fair to the pup to be tossed from one home to another.

My mother has had 5 shelties and I find they are the most how would you say it "snotty" dogs I have met.(in my opinoin please sheltie lovers don't get mad at me :0) ) I have had my beagle for three years and they still snap at her, I have had my cat for six years and if it gets to close they will snap.
So in all fairness to the puppy make sure your sheltie can at least tolerate the pup. Jumping from home to home might be stressfull to a pup.