Your timeline doesn't make sense to me. However there are always exceptions. In your introduction you state you're attending the Scout Sniper Selection Course in January, and you just recently stated that you're coming up to your 3 year mark in the Marines. Your chain of command is actually going to send you through Selection with less than a year left on your contract?
Plenty of stuff on the board already about the different branches of SOF. You question about timing etc, can easily be answered by a recruiter. Just like in the Marines, it's better to exhaust your immediate resources before reaching out for assistance.