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So I've always got a few projects to paint, and after finding a really good deal I came into possession of a substantial Slaves to Darkness force.

Due to some issues during the trade I also ended up with some Khorne Skullcrushers I hadn't really expected.

This started out as a pure Slaves to Darkness painting project but now I'm looking at other stuff to supplement my unexpected acquisition - I actually haven't gotten the rule book yet but from reading around as I understand it I can mark my warriors to Khorne and use them in a Khorne force, but can I do it the other way and just have a few Khorne units in a Slaves force?

I have

40 warriors (still working on loadout, 20 sword and shield and I might change up the rest)
2 chariots
1 sorcerer on foot
15 knights
3 skullcrushers

You bring up to 20% of your points as allies--these factions will not be part of your allegiance and accordingly not benefit from it but are a part of the army and any abilities outside of allegiance will affect them normally (command abilities, banners, etc.). If you are going khorne-focused I'd strongly recommend allying in a bloodsecrator. Alternatively everything you have can be run as part of a khorne army (with the appropriate mark) barring the sorcerer which can be allied in, so you have two options.

I will note that Slaves to Darkness is not as strong as an allegiance as Khorne but it has the advantage of being able to take multiple kinds of marks or no marks (which matters on certain units). If you aren't going to utilize that you will likely be better off going full Khorne.

Wrathmongers buff everyone. Even your enemy if not careful. Stuff like bloodletters and bloodletter hero have built in auto buffs too.