Well it is hard to give any suggestions because it is hard to understand what you want to do and why.
Cutting off power to a shield is not a wise thing to do if the chips on that shield have connections to the arduino. Feeding a signal into an unpowerd chip is a good way to destroy it or cause what is called a latch up where nothing functions when the power is restored to half the circuit and you need a power down of everything.
I am also looking to switch the current to another device (USB Host Shield )
First you switch voltage not current. Next are you talking about two USB shields?
I am unable to guess where would I place the positive and ground connection of the device which i need to run from the relay.
Guessing is never a good idea in electronics. Look at this page to see how to wire up a relay.http://www.thebox.myzen.co.uk/Workshop/Motors_1.html