According to picano, the best is 72? Guarenteed for no errors, but I feel that all mine are turning out worst compared to others I've seen.. (or at least from memory)
Are there known characters that allow for higher index's?
I really don't know, but you can go ahead, and index higher. It isn't recommended since it gives off errors, but if you'd like. DragonArts CSPs look like they're more than 72 colors.
We're working on (erm... I'm working on) a master CSP list to portray the maximum sizes that Indexing is allowed at. It will release along with the HD Youtube Tutorial 'CSP creating in GIMP' next week.