Each faction lists which other factions can be taken as allies. For the Slaves to Darkness, that includes Hedonites of Slaanesh.
Below a faction’s Pitched Battle profiles table, you will find a list
of allies that can be taken in an army from that faction (see 27.1).
The allies list will detail which factions you can take allied units
The "General's Handbook" Page 9, pitched battles table 1, states that armies in 1000 point battles are limited to 0-200 points in allied units, while in 2000 point battles, the limit is 0-400 points for allied units. That must be the origin of the 20% idea.
As for the second statement, the (August 2021) Errata on the Hedonites of Slaanesh 2 has the ...