> what do you recommend reinforcing silnyon with
At the minimum I use a full hem of silnylon, so the snap goes through 3 layers. Some of those snaps are tough beggars, especially in the cold. Sew at inner and outer folds.
Sometimes I use a hem enclosing a strip of something like 210 denier coated nylon, sewn at the inner and outer folds. The weight of a strip of 210 denier nylon is not great, but it adds greatly to the life of the item. Note: use coated nylon cut with scissors for this: no melted edges. In the long term the melted edges on nylon fabric are highly abrasive on their surrounds! Uncoated nylon would be silly here.
If putting snaps on netting - eg tent doors, I reinforce both sides of the netting with silnylon so the load goes on the sil rather than on the netting. I KNOW that netting is not strong enough. Don't ask how. :-)
Yes, you can surely use size 20 instead of size 16. Given the chance at a fair price, I would stock both sizes. You would be surprised where you can use them around the house as well.
Now, when would you use a zip rather than some snaps? I would always use a zip rather than solo snaps when I have to block serious weather - rain or snow. That's on tent doors for instance. I thought about using a zip on the ponchos and didn't - but note that I have both snaps and Velcro on the storm flap there. The snaps are not 'solo'. On a pack I might use a zip as well.