The rack deadlift, very simply is a deadlift that is done out of the powercage. You basically "rack" the bar on the pegs and deadlift it from there. It is also called a partial deadlift.
Reverse grip benches are just benches w/ an underhand grip. So instead of your nuckles pointing towards your head, the heal of your hand will.
I guess, but I find that reg benches hit triceps pretty hard, and I can personally use more weight on reg benches. But if you're stronger - by all means do them. I dont
Reverse grip bench is good, just be careful if you have no spot, because it is hard to unrack an appreciable weight without a spot. And if u can't use much weight, just might be a weakness that could be strengthened.
brian