I'm a bit late here but aren't leg extensions typically what we would classify as an assistance exercise? I would label the glute ham stuff as well except far more useful since most athletes tend to be quad dominant and ham weak.
the Sissy Squat. Interesting exercise. It reminds me of going from stand, back bending into a bridge and standing up in gymnastics. Balance might be tougher since it isn't done on the flats of the feet.