cel bags

No, don't get mylar for cels. They'll stick to the bags. The 'crystal clear' type that you mentioned are polypropylene bags. I use them for most of my cels, though, polyethylene works as well. In some cases, since I have large PE bags, I store the cel and the sketches in those, but separately bag each individual sketch or cel in either a PP or PE bag.

I personally prefer PE bags for my paper stuff. I love that they're easily foldable and don't need to be trimmed like PP bags.