Do Privates have to stand at parade rest and bring a battle buddy when speaking to a non-commissioned officer outside of OSUT?

You always stand at parade rest for an NCO, and still for a more senior NCO once you work your way up to one yourself.

As for bringing a battle buddy, typically not... unless one of you truly fouled something up. In the worst cases, you may even be bringing a number of the NCO's you fall under to stand before the 1SG, SGM, CSM, etc.

Damn kid, what you have to do in OSUT, and how a real unit operates is often as different as light and day. Since you seem to be in OSUT, instead of worrying about how real units are, why don't you worry about graduating into a real unit! IN my units, most of the time you would be a parade rest in front of me, maybe with you NCO chain of command depending on why you were in front of me.

You always stand at parade rest for a NCO until they tell you otherwise. You don't have to have a "battle buddy" but each NCO may be different on that. Some might tell their soldiers to bring someone else with to cover everyone in that room in case some accusation comes up later. Some commanders or 1SGs might have someone else in the room just so there's someone there in case an issue arises later. That's why you have to have one in IET when you talk to a DS - to protect the recruits and the drill sergeants.