Rebar with 1/4" or 3/8" for a base will work fine. They will get shot, they will shrug off most pistol. You should have a spare or two of everything placed downrange, props, sticks, plates, "hardcover", all of it, nothing sucks the life out of match like a long delay trying to cobble something back together. Make sure your stands are long enough to be driven well into the ground so as not to wobble and come loose.
Do not tilt falling plates, they probably end up tilted anyway and be a pain to reset. Do put a tab on the back of the plate's base or front of the stand so they fall rather than turn.
FWIW I've experienced a lot more ricochets off poppers than falling plates, it is however a fact of shooting steel that if you do it enough you will have some. Generally 21' is as close as any competition I know of allows steel to be engaged, you want a physical barrier to keep the competitor from getting any closer than the minimum if it can be engaged from there.