Guppies are harmless enough, but they prefer warmer temperatures than newts do. It would be best for the newts to allow their tank temperature to drop as low as possible in the winter, and the guppies won't like this (or could die). I have some white cloud mountain minnows in some of my newt tanks, and these are about the only fish I would recommend. They are small, harmless, colorful, and cold-tolerant.