Runts of the litter do not nec. stay small, they are just behind for one reason or the other. My pup's litter had large (17.5" -- 35 pounds) to two siblings who barely made 13". You want to look at the whole litter and see which are smaller but not nec. the runt.
Words of advice since you asked -- shelties can and do RUN. They will bolt and be gone. You will want to be even more diligent about the gate, keeping the dog on leash, watching around doors, etc. Other word of advice, either a Ruffwear Webmaster harness (it has 2 belly straps to help prevent slipping out) or a martingale collar for the leash. You cannot use a regular collar for walking purposes. Be sure to attend classes because it makes for a better dog. Chip and register the chip. Feed a lower fat diet.