Ok, I'm going to go old school. You don't need any fangle-dangled machinery or tapes or oojamaflips. We've been sewing since before these things were invented (although not denying that they can make life easier).
1) Wash and dry your fabrics how you will when they are made up into garments (ie, same temperature and if you are likely to tumble dry them, do that as well). If any are dry clean, wash them anyway. I never take anything to the dry cleaners, it all goes in the machine, on a cool gentle wash if needs be.
2) Pin the pattern well and in the seam allowance only, (so pin marks don't show). Do at right angles to the seam allowance in short gaps 1-2". Use a single layer of fabric if needs be (but make sure you flip the pattern for the other leg else you'll end up with 2 legs the same! Although to be honest, I don't consider viscose to be slippy!!!
3) Pin the fabrics together as above (which also makes it easy to remove the pins when you are sewing the seams) and you can tack them together for extra security if you so wish. I think an ordinary foot will be fine. I rarely use a walking foot and I have sewn plenty of garments made of silk satin.
4) Do a double row of stitches for the crotch, one on the seam allowance line and another a couple of mm next to it in the seam allowance (for security, ie, should the stitch break).
4) You don't need an overlocker. Yes viscose does fray, and the seams will need finishing to prevent this, but you can used a zig-zag stitch or if you have a fancy-shmancy machine, you can use the overcast stitch (but it is heavy on thread usage). Alternatively, you could make the seam allowance 2cm (usual is 1.5cm), pink the edges, if you wish,then fold them under and stitch.
5) And most importantly, don't overthink it. Sewing is sewing whether it is clothing or a quilt!!!