I have a Brother ULT & a Pfaff machine. I use a medium tear away on the bottom and a water soluable on the top for towels. If they are too thick for my hoop, I hoop the stablilizer and us a spray adhesive to hold the towel. For tee shirts or knits, I usually use a "ball point" needle - Organ size 11. Try not to stretch the tee shirt. Also there are several iron on stablizers for the back that keeps the shirt from moving. However, a design that is not too dense works best.
Good luck!
SandyGail