This idea is awesome! The only thing I want to add is, Genestealer Cult is great for representing a cult of Slaanesh, because their higher Initiative of 4 across the board represents how these cultists have the Mark of Slaanesh. I love it because it's a closer fit than even the Chaos Space Marine list allows for Slaaneshi Cultists. You basically invented the Slaanesh Daemonkin faction for SW
(An aside - Initiative, atm
is still a little underappreciated. It's a great "utility" stat that has so many uses in SW
I also agree with the Operatives issue above. Let's say you consider my feedback on specifically a Slaaneshi cult, because the mechanics fit the fluff so well. In that case, a Purestrain Genestealer could represent something akin to a Slaaneshi Chaos Spawn that is akin to a Daemonette. Pic related, imagine the Purestrain proxy as the figure on the right:
Aside from adhering closer to fluff and sportsmanship, the other benefit is that, imho
a Purestrain is more useful than other Operatives, due to when you would actually use an Operative in SW
:A campaign rules. I'll just mention this because not everyone has the rulebook yet, but "hiring" an Operative for a game costs a Prometheum Cache...which is the prize for you doing a mission in the first place. You get a Cache if you lose, 3 or maybe 4 if you win big, so when you have to spend 1 just to bring an Operative, it's a hard thing to justify. The exception is if you have to do a Rescue mission, when you have to sneak in and rescue a captured squadmate. In that scenario, an Operative is a great choice, and imho
I think the Purestrain is a great choice for that. You want something that moves fast (he runs/charges 18") and is very unlikely to sound alarms (usually by firing guns, pistols or noisy chainswords...he has none).
In any case, the idea of a Slaaneshi cult with Genestealer Cult rules is just so cool, nice one!