Welcome to Librarium Online!
Maybe IM missing it inthe codex, orsomething, but is there anything for slaanesh that keeps the daemon packs from scattering? Can any unit summon them?
In answer to the first question, not for slannesh. The only thing to stop daemon scatter is a daemon icon, which is undivided only.
Just remember daemon summoning is pretty tame next to normal deep striking. Normally you can survive landing just about anywhere.
As for the second question. Only a model with an icon of slannesh or of chaos undivided can summon slanneshi daemons. All units with the mark of slannesh can claim a free icon, but you really should model it so your opponent knows who is carrying it.. it's relevant if he takes a vindicare assassin or something.