I tried myself to come up with an explanation for this:
Maybe the difference between Rahi Control and Insect Control is simply that Rahi Control allows to only control a few Rahi at the same time, but any Rahi, while Insect Control allows to control a lot of Rahi, but only works on certain species and maybe also only on one species at once.
Thus you could maybe tell a dozen Nui-Rama what to do with Rahi Control, but to make a whole hive of thousands of Nui-Rama do your bidding you need to use Insect Control.
This still leaves open the question of what defines as an "insect" in the Matoran Universe, though.
That needs a more specific definition of "physical harm", I guess.
Otherwise since the Rahkshi powers are basically the powers the Makuta wield I always considered this to be simply a trait of Antidermis - even more so of gaseous Antidermis. I mean, try to hit gas with a sword. In solid form I always thought this power simply granted greater durability towards physical hits, much like a natural armor.
I'd agree with that. After all it's adaptive armor, so maybe it adapted to allow Tahu to wear the golden armor over it?