I believe that his cap hit includes whatever is guaranteed and whatever portion of his signing bonus was spread across his deal. So I would assume it is more like a $10M cap hit instead of $5M, even though I don't know the actual numbers to legitimize his cap figure.
I think Freeney would actually play pretty well in a 3-4 scheme. He and Mathis both rely on speed moreso than brute physicality. I think this transition to the 3-4 will be a good thing for your team, assuming you still throw the majority of downs.