Upgrading lighting in hexagonal tanks can be a real pain. If you can fit in a Compact Fluorescent rated as high as your plastic lamp cover can take safely, it should work fine.
That said, I recently read an article stating that the
WPG guideline works only for tanks over 20 gallons. They recommended 72W (out of T-8 lamps) for 10 gallons rectangular tanks.
If you stick to the low end low-light plants, even a 11W Compact Fluorescent rated at 5,000 to 6,500K would suffice. I am growing (slowly but healthy) Anubias barteri species, a native yet to id plant, and Dwarf Sagitaria in a standard 5.5gal with a 11W CF at 6,500K.
Pepetj
Santo Domingo