For the T-shirts, yes. You don't mention what sort of fabric the pants are, so I can't make a recommendation there. If they are polyester, then yes do so. If they are Docker type, then no. Maybe you could take the pants to a fabric store and ask the advice of someone there. They would be able to give you a better assessment for what you need than me, sight unseen.
Cut the garments apart first, leaving the pieces whole, then stabilize. You will get more usable fabric from a smaller 4" square.