Mono-Slaanesh is fine, probably the second best option after mono-nurgle and slightly ahead of mono-khorne. Possibly even better than mono-nurgle.
Here's a list I'm thinking about for a compy 1500 pt event:
Herald of Slaanesh on Chariot, Unholy Might
Herald of Slaanesh on Chariot, Unholy Might
Herald of Slaanesh on Chariot, Unholy Might
6 Fiends, Unholy Might
6 Fiends, Unholy Might
6 Fiends, Unholy Might
12 Daemonettes
6 Daemonettes
6 Daemonettes
Daemon Prince with Mark of Slaanesh, Aura, Pavane, Gaze, Iron Hide
Soul Grinder with Tongue
(Normal drop is the heavys, fiends, and 12 daemonettes first, then heralds and 2x6 daemonettes second)
Now, the list would benefit from replacing the daemonettes with plague bearers, and replacing the heavy support choices with Flesh Hounds perhaps. But it's not bad.
As an aside, Skarbrand is the Exiled One, it would be unfluffy to have him in a khorne army. Having him leading an army of slaanesh (Khorne's old rival) makes perfect fluffy sense!