I just need to point out that my little joke above was(hopefully) a very extreme example. I sincerely hope that the people who might join in our RP will have the sense not to try starting a relationship between someone like Sirius and a character of their own devising. Especially if they happen to be playing him or her as well.
In fact, I might be tempted to make Cassandra and Sirius enemies in this RP, just to alleviate the potential for problematic situations.
I have only one objection to make on the subject of the original characters being NPCs, if, by NPC, you mean, "Anyone can play them." That would surely lead to confrontation. If you mean "Only the GM plays them", which I'm sure you did, then I have no problem with it.
I personally suggest setting the RP in the Harry Potter timeline, mainly because James and Lily Potter are the only ones missing, and I haven't really seen anyone wanting to play them, anyway.
Plus, if the timeline is set right after Goblet of Fire, then things are nice and exciting, too.