any way to restrict to newsstand only in guided access mode?
It may sound counter intuitive, but each "Magazine" in Newsstand is actually an App, and the Newsstand "App" is actually just a standard folder that's been messed about with a bit at the OS level to make it appear and function a bit differently.

As such, with Guided Access being restricted to single app scenarios, you can't do what you wish, because when you are viewing the Newstand App you are actually just sat in a folder at your home screen - A double slick to bring up the multi-tasking bar shows that the left most screen which is normally associated with the Homescreen page/folder that you last launched an app from is showing Newsstand still.