What and where are the rules that govern mixing units of different factions into one army? (AoS Chaos demons and Slaves to Darkness)

Question

I'm starting to get into the hobby, and I wanted to make a mixed StD and Slaanesh army, so I read the rulebook and battletomes for both factions, searching for the right way to do that, and the only thing I came across with is the 1 out of 4 allied units rule (AoS Core rules, page 34.) or to make a Grand alliance army (for which I don't own the book).

A friend of mine is convinced that Allies are limited to 20% of the total points of your army (in addition to the 1 in 4 rule), but I couldn't find that written down on the actual rules.

Also, I found the following comment on the rpg.net forum:

If you're running a Slaves to Darkness army, it's just normal ally rules same as any other army. Allowable allies for an Slaves to Darkness army are: Nurgle, Khorne, Slaanesh, Tzeentch, Beast of Chaos.

If you're running an army for one of the four chaos gods, any Slave to Darkness unit that can be given a mark (it says so in their warscroll if you can) can be natively included in your army (not an ally) if you give it the appropriate mark of chaos. So if my army allegiance is "Nurgle" I can take Chaos Warriors with the mark of Nurgle as a regular part of my army, since the mark of Nurgle grants them the Nurgle keyword.

I've looked around the boardgame exchange, and related answers would indicate that both my friend and the rpg.net commenter are wrong, but I'm not 100% sure.

Can anyone point out if these are actual written rules for AoS and (if that is the case) provide a quote or the name of the book/page where I can find it?

Answer 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 following rule:

LEGIONS OF CHAOS: A Hedonites of Slaanesh army can include coalition units (see below) as follows:

• 2 in every 4 units in the army can be a coalition unit from the Slaves to Darkness faction that has the Mark of Chaos keyword. Those units must be given the Slaanesh Mark of Chaos keyword.
• 1 in every 4 units in the army can be a coalition unit from the Beasts of Chaos faction that does not have the Tzeentch keyword. Those units gain the Slaanesh keyword. A Hedonites of Slaanesh army cannot include coalition units with the Khorne keyword.

Coalition units do not count towards the number of Battleline units in your army. However, they do count towards the maximum number of Leader, Behemoth and Artillery units in your army. Coalition units cannot be generals. In addition, Coalition units are ignored when determining if the units in your army are from a single faction.

Designer’s Note: Coalition units are not allied units, so the limitations that apply to allied units do not apply to them. This means that coalition units can be given one of your army’s enhancements, as long as they have the correct keywords or are of the correct unit type needed to receive it.’