I think having some sort of supporting structure underneath the skirt would look better. You might be able to get away with properly darted and shaped fabric with stiff interfacing on the underside, although I like your idea of making it with hoops underneath. Maybe even just some horsehair braid in the hem would work to poof it out enough, I don't know.
For the hat, I think stuffing might make it a little hard to wear, just because it would end up being so top-heavy or make it sit funny on your head. Again, I think a light wire structure on the inside might be your best bet. At least some support for the sticky-uppy parts,, anyway.
Sorry; that probably wasn't very helpful but I hate seeing threads die unanswered xD
No longer cosplaying, sorry :\ Have fun!