They like to be able to spread out nicely without having themselves blown sideways. Think of them as tiny versions of your nem---what would make it happy will make these guys happy. They reproduce like nems, by dividing, and will slowly 'walk' a few inches (about 1/2 a day) to reach a more advantageous spot. With shrooms, always watch your ph, and your alkalinity. They like a ph of 7.3-8.3 and an alk of about 8.3-9.3, salinity of 1.025 give or take .001, and are generally very good growers. Do NOT get an elephant-ear mushroom: they're dangerous to small fish. They're kind of brown-green, large, "hairy" and they close up on intruding fishes and eat them. All other kinds including some 'fuzzy' ones are perfectly safe and a great deal of fun.
Visit the corallimorphian forum (that's shrooms) and tap their expertise, too.
sometimes you'll have a shroom leave its rock and blow about the tank like an angry nem. With a nem in there, you'll want to catch it and pin it down fast. There are various techniques for doing this: rubber-banding a piece of netting to a rock with the stray inside is classic.