Serger threads are not mostly 100% polyester. Most often they are cotton. However, they are generally not long staple cotton. Either of the sizes you mention are fine for serging because of the multiple threads. If they do not pass the 'tug' test I would not quilt with them. I do not piece with serger thread; however, there are people who do. Serger thread loses tensile strength because of its low twist. I do not have an embroidery machine so I cannot comment on that aspect of their use.