If we could move down to 25 that's a different kettle of fish. Carpenter looks a lot better at 25 than 18, so does Whitner, so does Wimberly, Lawson, Winston and Watson. But 25 ain't 18. 18 would be ok for Jackson or Holmes but I can't see us going WR in the 1st.
If we were stuck at 18 and felt we didn't want to take one of the LB's because it was a reach I'd probably pull the trigger on Antonio Cromartie and put him at FS. He has the athletic ability to be a top 5 pick and the size to match up at FS. That could be a very sick secondary and Cromartie would give us automatic extra depth at CB.