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if an IG army takes a daemonhunter HQ, does it still have to take an IG HQ as well? Or does the deamonhunter HQ satisfy the required for the FOC?

The allies rules in the DH codex specify that the army still has to include any compulsory choices from the parent list.

What Insaniak said.

Compulsory choices MUST come from the parent list.
In the OPs case, this means 1 HQ and 2 Troops MUST be from the IG codex before he can even add in ANY GK/DH allies.