2
Yes.
Each faction lists which other factions can be taken as allies. For the Slaves to Darkness, that includes Hedonites of Slaanesh.
25.8 ALLIES
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
from ...
1
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 ...
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