Barkley was an undersized power forward who was a rebounding machine. He was dominant on the interior glass and could step out and hit the three. Durant is a do-everything small forward.
Generally, a "tweener" is someone, usually a forward, who is still small to play power forward but not quick enough or not having enough range to play small forward (think Tracy Reid, LaQuanda Barksdale, etc.).
I do not see KML as a tweener or a hybrid of Durant and Barkley. To me, she is more of a hybrid of Katie Douglas, Kara Lawson, and Seimone Augustus, but even that comparison is not 100 percent. She really is a unique player.