I don't see any size restrictions with snagging strike. Can this be used against any opponent, or is it intended only for similarly sized enemies? Can a halfling use it on a purple worm? What is RAI?
There is no size restriction on imposing the flat-footed condition. I don't see a problem here.
Neither do I. The feat says that the fighter throws in some grappling moves, but that is largely just fluff. You aren't actually using grapple or athletics with the feat, so shouldn't be subject to the same restrictions.