It would be fine to under-line that rod if you were using a sinking line. You could also use the WF6F on it, but you'd probably get annoyed (and tired) trying to cast it. A full flex rod (or even mid-flex) would probably be ok to under-line in general too. A fast action rod just isn't conducive to it!
My passion for catching fish is eclipsed only by the fish's passion not to be caught.