I do lots of sweatshirts. I always use a fusible cutaway on the bottom. If you are doing a heavy design (something that I frequently do on sweatshirts), use a second layer if you choose to use a polymesh stabilizer like I usually do. A single layer of polymesh only holds about 8000 stitches per 4 or 5 square inches, and most of my sweatshirts have more than that number of stitches. I do not use a topper, but some do. If you do, don't use a traditional tearaway but instead use a washaway topper. The tearaway won't all come away!
Pam (getting ready to embroider a Halloween sweatshirt with the word "Don't make me get my flying monkeys!")