Can you cast shrink item on body? I believe so.
Can cast shrink item on zombie?
1) Yes
2) No. A zombie is a creature, not an object.
Oh, not the question I was expecting. But eh. I might as well ask that one then: If you cast shrink item on a corpse, and then cast Animate Dead (or heck, even just a Raise Dead to make it living again), what happens?
The easiest answer would be that it is canceled out and the creature returns to full size, since it becomes a creature and thus no long a valid target for the spell.
Which could be nice in its own right. If you embed the onyx into them before shrinking them, then you could just keep them in your pocket. It would be like pokemon. Romero, I choose you!
Someone else answered the way I would, "yes" and "no".
I had a necromancer who had a pocket full of shrunken skeletons and corpses. He would toss them out and after they hit the ground they would regrow so that he could animate them. Good times.