konichiwa,

as a fan cel artist, i'd have to recommend the 
pens that i am currently using:  faber-castell 
india ink pigment pens.  once you get these inked 
on acetate, it's extrememly difficult to get them 
off once dry.  they are acid-free, waterproof, 
and lightfast as well, so they'll keep for a long 
time.  i've never had any cels signed myself, so 
i'm sure that anyone who has might have another 
suggestion.  good luck,

haruna