I geniunely believe 75%+ of SW aquariums are under-'rocked' and really could use more. I love taller, more structural rock formations, rather then a "pile of rocks" look. Some of my aquascapes have even breached the water surface. Pounds of rocks really could mean different things. A 100 pounds of dry base rock, is much more then 100 pounds of dripping wet live rock.
Since it's a 55gal and you really won't (shouldn't) have any larger more active fish, the smaller fish will enjoy darting in and out of the rocks more.I would starting with at least 80lbs, Look for more interested rock pieces that will help you build interesting formations, rather then just round boulders.