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?