If I requested an installment plan with automatic debiting, do I still need to mail my first payment in with 1040-V? I set my date for 16th, which has already passed.

Yes, you should go ahead and make your first payment.  You should also continue to make monthly payments to the RIS until you have received confirmation from them that the Installment Agreement Plan has been established.  It can sometimes take the IRS a couple of moths or more to get things set up.