I've been told that new shelves can't be added to a filer when not filled with disks. Is it possible to add a shelf that has half its slots filled with disks?
Yes, it is possible.
Yes, It's possible to purchase a shelf that's half populated and then add disks as your storage needs increase. Please contact your Sales Rep for details.
Technically you can have a shelf with as few as three drives (if I remember the number correctly), but on most occasions it is not feasible from a commercial point of view (half-populated shelf will be cheaper / the same).