Using a tourniquet is risky business. Unlike indirect pressure (pressure points), which cuts off the blood flowwing through a specific artery, a tourniquet affects flow through all the blood vessals in a limb.
If a tourniquet is not applied tightly enough, it will cut off the flow through the veins, but not the arteries. This can INCREASE the ammount of bleeding from a wound, which is obviously not the desired effect.
Also, if a tourniquet is used in the wrong place on a limb, nerves can be damaged.
Given these risks, I would opt for indirect pressure to control a major bleed in a limb.
Finally, we live in a increasingly litigious world. If you do things you aren't qualified to do, be aware that you may leave yourself open to litigation.
Chris