Can I just share my iCloud storage with one of my designated family members and not all?
Can I just share my icloud storage with one member of my family and not all 5
Sorry, no. However if some of the family members have their own upgraded plan (i.e., more than the free 5GB) they can stay with that plan and not switch if they don't want to.
Also see: Share iCloud+ with your family - Apple Support