Fussy Wooly Nylon thread I would use in the upper and lower loopers for the lycra fabric.
Fussy Wooly Nylon
A textured, yarn-like thread that creates a very soft, but strong edge or seam. Use in the loopers of your serger and/or the needles. Wooly Nylon may also be used in your sewing machine. The thread has an elastic value. Wooly Nylon is perfect for rolled hems and edges, ruffles, swimwear, aerobic wear, kids' clothes, knits and lingerie.

For most fabrics that tend to ravel I use wooly nylon in the lower looper, the upper looper covers it but prevents thread from poking out.

the thicker threads always go in the loopers, thin serger thread goes through the needles. when doing inner seams only the second needle thread will show so that should match your material, all the others can be coordinating colors or just dark or light.

Many people like wooly nylon because it stretches fine and stays fuzzy when not stretched to fill in the stitch more. It won't effect the stretch of the stitch, what effects the stretchability of the stitch is the setting your choose. so consult you manual for a stretch stitch.

when doing stitching that will show such as rolled hem or flatlock, then the looper thread will matter. My personal favoite is for decorative rolled hem stitching is YLI Pearl Cotton and YLI Metalic threads.

Hope you are enjoying your new machine !!!