I have been told you need mighty deep pockets and nerves of steel to short pennies! And mighty deep pockets if the stock turns on you and you run out of cash your toast! Your brokerage house can set you up I remember reading here that you will have a cash on hand requirement 25 k I believe. If you want I can look up the old post.
I use interactive brokers a lot also, but I never short a penny stock. If a PR comes out right after you short, you can loose more money than what you have in a matter of minutes. You only have till opening to wire or transfer funds or they sell what ever you have starting with your best stock. Many shorters end up in BK. Looks easy, but it only takes once and your financing is devastated.