Each block of sand will house one cactus plant so plan accordingly for how large you want your cacti farm. Cactus cannot be placed next to any other block. You do not need sunlight, water, or tilled ground in order to plant cacti.
Use the joist You never want to try to mount a heavy bag directly to drywall. Same goes for a drop ceiling tile or support frame. Drilling directly into a popcorn or plaster ceiling is a no go as well. Even with the use of anchors rated to hold a lot of weight, it just won't work.
Simple - 9 lbs per square foot. If your chair does not weigh over 9 pounds—you can hang one on a joist for every foot of length. This is based on a attic floor which is designed by code to support 10 pounds per square foot and then subtracting 1 pound for ceiling weight. You should have no problem with this.