I have made quilts with t-shirt type knit without the stabilzer. If you use a sheet in the middle, then pin or spray baste the top to the sheet and backing you should be able to quilt it if you are careful. I think the backing fleece is more likely to stretch out of shape than the t-shirt material. I would cut the t-shirt pieces as small as I could without disturbing the design on the shirt. Try it first on similar material and size.