It's a good idea. It certainly doesn't hurt. After all, the bench presser has to work less immediately before an attempt. In a competition, any aid (such as a bench shirt) is welcome. (If you are doing a heavy single bench press at home, please always have either a spotter or two or do them in a power rack with properly set safety bars.)
To properly perform an elevated Smith machine squat for maximum effectiveness and safety, follow these steps: Set up the Smith machine bar at an appropriate height on the rack. Position yourself under the bar with your feet shoulder-width apart and toes slightly pointed out. Engage your core and keep your chest up as you unrack the bar and step back. Lower your body by bending your knees and hips, keeping your back straight. Go as low as your flexibility allows without compromising form. Push through your heels to return to the starting position. Repeat for the desired number of reps, focusing on proper form and control throughout the movement. Remember to start with a light weight to practice the movement and gradually increase the load as you become more comfortable and confident in your technique.
The squat works many muscles in the body in some form. The primary movers are the hamstrings, quads, and glutes. Assuming you are doing them correctly with your knees out and squatting below parallel, you should be using your hip adductors as well.