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Can a primarch be chosen in a leviathan force org with imperial knights? Just the primarch?
Also, can Gahlan Surlak be chosen as a compulsory HQ in an allied detachment?
He is a support officer, but it says "may not be a compulsory hq if chosen in a primary detachment"
I want to take knights with a 20 man tac angron and 10 red butchers. Is this possible?

I would say no. The allied chart doesn't allow a Lord of War and the allied HQ has to be a compulsory

I never noticed it said he was a support officer.

As long as it's imperial knights, does it ned to be a leviathan detachment? An Imperial knights primary detachment, with a primarchs chosen allied detachment should do most of what you want.

Isn't primarchs chosen a primary? Wouldn't angron have to be my warlord? can I takr both?

The allied contingent only allows for a single low, if i reca . You wanted to run multiple knights

None of what you listed can be taken in a Leviathan detachment.

The Compulsory LoW for the Leviathan detachment must be either an Engine of Destruction (a vehicle with 9 or more Hull Points) or a Great Beast (a gargantuan creature with 9 or more wounds).

Neither a Primarch nor any variant of Imperial Knight fit those criteria.

The best way to run World Eaters with Knights is use the Questoris Knight Crusade Army List allied to a World Eaters Primary AoD (or RoW) that allows a Lord of War slot for Angron who would then be the Warlord.

As for your second question:

Absolutely yes. Gahlan Surlak can certainly be the compulsory HQ in an allied detachment, because an Allied Detachment is not the same thing as a Primary Detachment and the wording specifically calls out only Primary Detachments.