Incorrect, sir. It seems likely that Ishamael founded the Black Ajah during one of his "free periods" at about the time of the Trolloc Wars. The Tower stood (relatively) unmolested by the Shadow for a millennium.

If it took a thousand years and the Shadow's most diabolical genius to crack the Aes Sedai nut, I'd imagine the Aiel, with their strong filial bonds in addition to the bonds of society and culture and cache of ter'angreal that gives them convenient access to ancestral memories of experiences from the AoL itself (id est: first-hand knowledge of the Shadow's greatest atrocities), would make an even-more difficult target for infiltration. Aes Sedai are far removed from the organization from which they claim descent and they are not a nation in the sense of having common ancestry (which counts for a lot; people tend to be more loyal to their family than to their coworkers, for instance).

Furthermore, if we're going off the assumption of "Ishy's Responsible For All of the Disasters of the Past Three Millennia at Periodic Intervals" as put forth by the Ogier in teh BWB, we can also assume with relative safety that he was way too busy chipping away at the Aes Sedai and Artur Hawkwing problems to spare much thought for other cultures beyond throwing them into chaos as a result of his plans for the two aforementioned parties.

He was always letting the Lord of Chaos rule, and that was his first prerogative. His targets were institutions that threatened to impose widespread order (the Compact of the Ten Nations, the Tower, Hawkwing's Empire), which would have allowed for a better-coordinated resistance to the Shadow.

The Aiel are fierce warriors, but he probably already believed them isolated and compromised enough, what with their retreat into the Waste and rejection of the Way of the Leaf.