When I sewed with fleece, I used a narrow zigzag stitch for the seams. This builds in enough "give" so that the seam doesn't break if the fleece is stretched.
Fleece edges don't ravel, but the seams can be kind of thick. A different way to do the seams is to overlap the fabrics and zigzag on top through both layers. I think a 5-thread serger can make a stitch like that too. This finishes all of the seams decoratively and they are only two layers thick.