What if the Grey Council died?(Babylon 5 RAR)

[Kojiro]

Were the Narn the first race for the Vorlons to experiment with giving telepathic powers to use as weapons?

It's never really detailed who was first but it was probably the Mimbari. Humans got modified (or perhaps activated) only recently. At a guess it corresponds with our entry to the larger galactic theatre as players and there's a decent chance the Vorlons interfered as species joined the stage. That said the Narn were at least partially occupied by Shadows in the distant past and fought a war against the occupiers (a running theme with them). Whether this was before or after Mimbar gaining Vorlon attention, I don't think any time frames are given. But I'd say the Mimbari were the first into space and the first (and most heavily influenced by) the Vorlons. But that's just a reasonable guess.

[lord Martiya]

Question that I have always wondered about, curious if anyone knows.

Were the Narn the first race for the Vorlons to experiment with giving telepathic powers to use as weapons?

Or were they the first younger race to develop mental powers and before this the Vorlons and their allies had never realized just how vulnerable to telepathy? The success of the Narn telepaths driving out the Shadows in the last war suggested to the Vorlons that telepathic ability should be seeded amongst the other races to create a new line of weapons?

From what I know (coming mainly from the RPG books), the Vorlon had found out that the Shadows are weak to telepaths in the Shadow War before Valen's one but too late to make use of it, and experimented with various races. The Minbari were the guinea pigs, with the original telepaths being too powerful for their own good (the weaker ones were on par with Bester, and their attempt to take over Minbar nearly drove the race into extinction. The descendants of the few telepath survivors were weaker because they interbred with non-telepaths).
After working out a process that made telepaths with non-excessive power, the Vorlon seeded telepathy into dozens of races, with the Narn being unlucky enough to have to fight the Shadows on their own homeworld and getting a telepath-killing weapon tested on as a last 'present' from the Shadows, who were running.
It's unclear if the Centauri were modified by the Vorlon or spontaneously developed telepathy (I don't know if Londo lied or not when Kosh emerged from his encounter suit and he said he saw nothing), but, also according to the RPG, human telepaths are somewhat more powerful than those of other races (how much is unknown, nor if this is on average or just the high-end human telepaths being more powerful than the high-end telepaths of other races) because the Vorlon tested a new telepathy-implanting process on them.