Please help with this. I have never truly understood the new way to select an army.

With the new chaos books out i'm looking at taking the warband primary detachment + auxiliary.

But i also want stuff from the FW books that state "this can be taken as a HS choice"

So, the question is how do i take these? Do i just take the units anyway without consequence or do i need a CAD from the codex? or is this just not allowed outside of friendly games?