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So in the index / codex it declares DG and TSons cannot use the units as they will be defined in a seperate codex. Does this include DG and TSons for forgeworld? Such as Hellforged? Or can I use a hellforged DG / Tsons unit without breaking detachment rules / legion trait?

Yes - the FW FAQ explicitly says you can. However the FAQ also says although they are HELBRUTES they cannot make use of Fire Frenzy.

FW FAQ says something about this. https://whc-cdn.games-workshop.com/wp-content/uploads/2017/11/imperial_armour_index_forces_of_chaos-1.pdf