An apology
I've been looking at the game files again, and discovered that according to those, Nihlathak is level 92, not 95 as I stated earlier. So I do hereby apologize for any inconvenience caused by my misstatement. Earlier on, I took my information on Nihlathak's level from the Arreat Summit, which is apparently not a reliable source of information. I took it over without thinking since I remembered him as being level 95. Apparently this was true in versions 1.09 and earlier. Note that this time I took my information from the game files as used by ATMA, according to which he was level 95 in 1.07, 1.08, and 1.09, and level 92 in 1.10 and 1.11.
Where does this lead with regards to charms? Well obviously he can not drop a charm with an affix of level 93 or higher, for example the hibernal small charm, nor can the charms he drops be used to reroll for such a charm. Of course if you would run him in 1.09 or earlier, the situation is different, all the more so for the lack of bugged vipers making that running much safer, there might however be differences with regards to how much damage his corpse explosion did then, I don't know, it might have been more and it might have been less.
In 1.10 and later, he ranks more or less equal with Baal's minions with regards to possible affixes on charms (equal with Lister; Ventar and Bartuc are level 93; Achmel and Colenzo are both level 88(I'm not sure, it might be only level 85/83?)) , although he drops far better than them, they have only one pick for an item, as opposed to his 5. Still, this returns Baal as the only possible source of hibernal and shocking small charms in 1.10 and later, and Diablo and Baal as sources for pestilent and flaming small charms. Small charms of craftmanship can be dropped by Baal, Diablo, Nihlatak, Lister, Ventar, and Bartuc and minions of the latter three(?).