1)Since the poured concrete walls are not plumb there will be some gaps between the concrete walls and the studs--say around a half inch. These gaps would allow the stud bays to "communicate" with each other in the event of a fire. Do these gaps need to be plugged? If so, what should I use?

2)On one wall the concrete wall ends at about 3 feet high once it rises above grade. The sole plate and house framing on this wall are inset about 3 inches from the interior concrete wall (in other words, the existing sole plate and studs are about three inches narrower than the concrete wall). I want the interior basement walls to be straight (and not "stepped out" at the 3 foot height). This means that there will be about a three inch gap between the new studs and the exiting studs from the 3 foot height to the ceiling. Does a gap of this size need to be plugged so the stud bays don't "communicate"? Could I just attach 1/4 inch plywood to the studs or what?

Thanks!