Honestly, I see no point in even semi-regularly running any formation without a safety or with less than two linemen plus at least 3 linebackers.
Without a safety, you're just asking to give up that big play.
As for < 5 linemen/backers, way too easy for the O line to shove people around and for there to be a quick audible to a run for 4-6 yards. Also, you need some big bodies to tie up the O linemen in order for your swift, athletic types to get to the QB. And even in an extreme prevent to win situation, why wouldn't you rush at LEAST 4?