1) You don't have to declare detachments before you write the army list, you kind of do it at the same time. If you decide you want to bring a battalion, you just bring one. As long as you're under the points limit you're fine.
2) Yea, that's a big part of the game, picking which detachments to run. You do not need to mirror your opponent, some armies are incapable of doing that. Like, I play Guard, and use a Brigade detachment. Most marine players don't have that ability, since their units are more expensive. Knights can't use anything but Super Heavy detachments since all their units are super heavies.
3) I don't know if this stuff is explicitly listed in the BRB, but it's how I've played it and how everyone else plays it as far as I know.
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