To keep your seam from bunching up as you start to sew, do not back stitch. Instead put the feed dog down and sew a couple of stitches in place, then put the feed dog back up and continue to sew the seam. No bunching, you can end the seam the same way.

Usually what the garment industry uses is the clear elastic. However, whenever I sew the shoulder of a knit garment I simply cut a strip of the same fabric on the lengthwise grain and apply it to the shoulder at the same time as I sew the seam together. I use the serger so the fabric is trimmed at the same time as the shoulder seam. This stabilizes the seam just fine. and is not too bulky.