No danger in using cork bark. It may look rough, but it´s actually rather soft. I´ve used it for years, and lots of other people have used for much longer with excelent results. You can cover it with java moss and the effect is quite nice.
I prefer it to stones breaking the surface cause the stone take a lot of water volume. Another very good option are bricks. They provide suffficient land area for an emergency, and holes for the newts to rest in.