I only stabilize the fabrics that need stabilizing. By adding a featherweight stabilizer like Pellon 911FF, you will add almost no thickness to the tees.
Be careful not to leave the iron too long in one place where you have vinyl decals or it will melt the vinyl. Have at least a couple extra tees on hand just in case. Once you've stabilized the tees, they are no longer true "knits" so you do not want to use a ballpoint needle to stitch them together. I like a Microtex needle because it is super sharp & easily able to pierce through the t-shirt & fusible interfacing.
Happy quilting!