Can You List in a Newly Created Category?

Yes. A category or subcategory becomes a fully public, functional leaderboard the instant its creation payment clears — there's no separate "opening" step.

It immediately appears in navigation and category dropdowns, and anyone (including the person who created it) can submit a listing to it. That makes creating a category a legitimate strategy on its own: you can be the very first listing in a niche you just defined.