Welcome to MAAST Noober!

Some say 2 lb/gallon is a must for live rock, but I haven't really heard that here. I ran my 90 gal tank with ~80 lbs live rock when I first set it up, then added ~20 lbs of dry rock and it has been just fine for me. All of my basic parms are 0 (even nitrate). I would recommend at least 50 lbs of rock and 85 lbs would be great (I'm assuming your 50 gal tank is 36" in length). If you're going to add more rock, get rid of the damsel first. They are tough fish, but once you add more rock, you'll never catch the little devil. Besides, the rock will provide all of the ammonia you need to get your water cycling.

As for sand, I don't think lbs/gallon is worth as much as average depth. As a general rule, I've hear less than an inch, or more than four inches (it depends on whether or not you want to have a Deep Sand Bed).