I use a soft mesh cut away for the back and the thin plastic wash away for the top and just as Barb said - I floast a piece of tearawy under the hoop - if I have a dense design or two layers of the mesh cut away in the hoop. I test my design on a piece of fabric that is close to the t-shirt material., too.
You've gotten some really good answers to your question. I just want to add that if your design is really dense, you can float a piece of tearaway under the hoop regardless of the type of cutaway that you use. Good luck and have fun!
WWS on the top, not need to be the one used for FSL, the light weight one works just fine to provent the stitches from sinking into the fabric and cutaway on the back. I personally prefer the no-show polymesh type as it is very light weith but it gives you the necesary stability and will not show thru the fabric.
Here is a link how you can embroider t-shirts and what to use. Good luck.
I always use a sticky paper underneath and wash awy on the top.always good result