there's no set rule and you don't need approval.
however, we always recommend that you play your first character for a little bit and get them active before moving on to your second. again, just a recommendation.
that said, you are more than welcome to have WIPs in your sandbox. for the type of character question, it'd probably be easier if you knew who you wanted and just asked about that specifically? there are so many options~
when you do want a second character, you'll have to log out and register a new account with a second email address. once they're sorted, you can connect your characters and have one "master account" and be able to post all your characters from that account.
that was a rambly answer so feel free to ask for clarification on anything XD