Java moss is very hardy, and grows best attached to an object such as a rock or driftwood. It grows very rapid. 6 to 8 pH, can tolerate low temps. I love it and my cory cats spend 90 percent of their time resting on it. If you don't trim it or take care of it, it can grow all over the substrate, which I personally don't mind. For your question on guppy fry, rotala is probably the best for them to hide, although it requires moderate to high light.