I'm curious how you can 100% guarantee me this?
It seemed to me that stalker basically said heavy skirmishers were bad, but in more polite terms. There is no niche. Pillager is not a niche, all units can pillage.
Scout isn't a niche, there is already a unit line for scouting that has much better promotions, and doesn't require horses.
Harasser, I don't know what this even means in terms of gameplay, but dealing small amounts of damage isn't a niche and taking hits poorly isn't a niche. Rather that just harass the enemy, other units types could just kill it.
I think I'm basically suggesting your idea but with bonus damage on open terrain, so that the units aren't useless in areas without forests.
On a somewhat related note, giving Mongolia logistics to start is a really bad balance change. See the Inca thread for evidence of how broken early logistics is. Mongolia is already an amazing warmonger (even with the relatively weak status of skirmisher units) and ignoring ZOC is much, much less valuable that attacking twice.