There isn't much for short grass. Dwarf Hair grass can fill in a beautiful carpet but even under intense light it can get 3-4inches. If you trim it it will grow back rather quickly giving it that natural feeling you want. Other really good short short plants that fill in would be Glosso or HC. I have hair grass, glosso and HC and the HC is Tiny and stays really short but is hard to keep healthy. Even in my tanks with 6WPG or more the hair grass can grow 3"+. The glosso spreads really fast and is low laying but takes longer to cover the substrate.