I've heard mylar bags taking off bits of paint, 
but I haven't heard anything about the other 
bags.

There's really no fool-proof way to keep cels 
from fading, although you can considerably slow 
down this process if you treat your cels well.  
Just keep away from light (esp sunlight) and 
high humidity (windows, bathroom).  If you're 
gonna frame, use uv-resistant glass and get the 
glass recoated/replaced every 2 years or so.

I just wouldn't recommend putting any kind of 
paper (even acid-free/archival mats, rice paper) 
directly behind cels because the paint will 
stick over time.  When people frame their cels, 
the cel is separated from the mat with a cut out 
cel bag layer or using archival corner mounts 
(which can just be corners cut from a cel bag).

I do what most people here do: Keep cels in bags 
and keep those bagged cels in 
portfolios/binders.  You should probably replace 
your bags every 2 years or so as well.  Most 
people in here use bags made of polypropylene.  
I keep a few of my special cels framed.

Again, most people would rather shell out their 
extra dough on more cels rather than storage, so 
just go within your means and check out prices 
in art supply shops.  It doesn't have to 
say "cel" in whatever supply you're looking at 
too.