Dan,
I would think that as long as it meets the minmum size requirements
to be a class spinnaker, it should be legal. I don't think it would
be smart to use that as your primary spinnaker though. And I wouldn't
use it as a back-up as well. If we had one I wouldn't use it for
one-design stuff. You should be able to hold a class legal kite just
as well with a full crew on the rail.
You are right though about having to have one of your spinnakers at
least a year old. This makes sure people aren't winning races because
they are buying new spinnakers every weekend.
Cheers,
Jason