Just investigated that and appears that 12" is the minimum length.
( I had a nice piece of QS oak that was about 11 1\2" long. I just glued a pair of runners to each long edge, ran it through to ultimate thickness I wanted and sawed off the glued runners on the band saw. Worked great!)
Could the runners be attached using the brown paper bag method to save having to saw them off, loosing stock? Hot glue maybe, or CA if they aren't going to have a stain applied.
I'd suggest using the CA glue + painter's tape trick to affix short pieces to a planer sled or just a flat board longer than the minimum recommended length for this machine. I got great results recently doing this for very thin boards (not short).