Pick up some 'medium' sized supers and try her on those...I honestly don't recall how big my guys were when they started taking on the huge monster supers. Look through the containers and find one that is mostly small to medium, give them a shot and go from there.
Once your Leo 'crunches' the worm she will swallow it 'the long way' so there really isn't too much to worry about as the worms, even the big ones, are slender.