Even rice paper will stick after prolonged 
periods of time.  Just stick a sheet of 
polypropylene between cels and sketches (or 
backgrounds) if you prefer to have them both in 
the same bag.  (I always keep my sketches outside 
the bag in my cel book.)

Try to avoid cheap (thin) polypropyelene 
bags/sheets, and go for thicker bags (~3mm or so) 
since the thicker sheets won't warp as much.  If 
you have the money, then go for mylar bags 
instead since they're much more durable.

Sketches are definitely NOT acid-free.  Sketches 
can also turn yellow after time, so sketches also 
need proper care.  For valuable sketches, I know 
some people back them with basic pH paper and 
keep them in polypro./mylar bags.