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For example, if I had a Blades of Khorne army, could I use a Slaves of Darkness unit (like a Sorcerer Lord on Steed) AND a Chaos Gargant? Or would the Allies part need to be limited to a single faction?

I haven't read the ally part yet, but those it state a restriction on the amount of allied factions? If not, you can take units from as many different factions as you can fit in

The only restrictions allies have: only the listed factions of your main faction and not more than the allowed points.

so you can mix allies as you wish

Excellent. Thank yous!

There are two ways to use allies in Matched Play:

The first way is to just play a Mixed Order/Destro/Chaos/Death army. There are no restrictions in Matched Play as long as they are the same faction. So the same as before. So if I want a Seraphon army but want to mix in some High Elves, Dark Elves and even some Dwarfs? Can do that no problem.

The second way is to take an allegiance and take the allies listed in the GHB2017. There is a cost limit to this./ Let's say I take a 2000 Point Seraphon army and want all those nice new command traits I will need to declare it's a Seraphon list, which restricts me to 400 Points on Allies and I can only draw them from the Stormcast Eternals list. If I go over 400 points, or draw from another list, I lose the Seraphon allegiance and become a regular Order army.

Do allies take up leader slots?

Allies count toward the maximum slots you have, but don't count toward the minimum.

Sorcerers with Blades of Khorne?
EEEWWWWW!

Blades of khorne can use any slaves to darkness that can take the mark of khorne and maintain the allegiance ability. (so everyone besides wizards basically can be taken)