I'm in a similar situation to you as I also have 
cels from many different series.
Currently for series that I only have 1 or 2 
cels, I have divided the cels into Miscellaneous 
sections based on the series name.
At the moment its Misc. [A-G], Misc. [H-N] and 
Misc. [O-Z]. I set up a little mouse-over to show 
whats in each section so people can look for (or 
avoid) stuff in that section.
I try to keep the number of cels in each section 
below 10 to minimise loading times.
Anyway, hope this helps.
- X