Lighting per gallon is very subjective thing.....It isn't very accurate as well when trying to get the right lighting for corals of all sorts.....It also depends on what type of mushroom you are trying to keep some are better suited for low light while some like Florida ricordea are just as good in high light.
What type of lighting do you have or are going to have for these mushrooms? I think if you have VHO, T-5, MH you can keep any mushroom you want. You will have to find the place in your tank where they thrive and leave them there......