The last applications post is where Bunny listed up the position of Prefect. I don't see any problem in completing the Prefect application if you're interested in Co-Prefect, and just specifying that fact on your comment.
The Applications Post is found here.
EDIT: Oh, and for those who were pointing out the fact that so few of our officials are Hufflepuffs or Slytherin, well.. I thought I might drop a little bit of a *hint hint, nudge nudge* here by mentioning the fact that we really don't have either house applying for any of the positions opened in that post.
~ Melinda o' Ravenclaw