To be honest, that entire watts per gallon rule is close to obsolete. Sort of like the one inch of fish per gallon of water rule IMO. It's a very broad way to find out how much light corals need. I think intensity, PAR, and coral placement is more important than watts per gallon.
But regarding the mushrooms, you should be fine keeping them under almost any light. They are very low light tolerant. Bear in mind however that if you get really colorful mushrooms that are used to a moderate to higher light environment, and place them under very weak lighting, they will most likely brown out. And when I saw almost any light, I mean any light - I've seen mushrooms kept in a 15 gallon tank w/ (2)9w PC's. For your 10 gallon tank, I'd recommend going for a 20" (1)28w PC fixture for mushrooms. HTH.