The biggest question I would ask myself would be 
whether putting multiple cels in the same frame 
would detract from or enhance each of the 
individual cels, as opposed to framing them 
separately and arranging them together. The cels 
are obviously connected by content. Would framing 
them within the same frame make them lose their 
individuality by forcing them too closely 
together? Or would they complement each other? 
You're the best person to decide that one.

I would suggest that you go to a real art gallery 
(not a craft store that has a framing service) 
and consult with the framer on their opinion. 
They're used to working with art, and would be 
able to give you good advice on mat color, frame 
type, care, and arrangement. If you don't have a 
personal relationship with a particular framer, 
or know who's respectable versus who's 
just "there", go visit their gallery and see what 
they have before requesting their services. A 
good framer should be willing to talk to you 
about what they do. Due to the nature of cels, 
and how they're not "normal" art, and their one-
of-a-kindness and expensiveness, I wouldn't skimp 
on the framing costs. It might cost twice as much 
as a craft store framer, but you get what you pay 
for. :o)

Good luck!