Can you clarify your exact setup? Do you have a 1:1 sync between SFDC and Marketo? When you say "below the requirements" do you mean Lead Score<70 and Behavior Score<10 should be in nurture? Or Lead Score>=70 and Behavior Score<10 should be in nurture.
Do you have your entire journey mapped out? I highly recommend creating a flow chart for the whole lead lifecycle, thinking about every possible path. Better to set it up all now holistically than piecemeal.
Is "lead score" being used as demographic score and is it separate from behavior score? In many scoring models, behavior score is added to demographic score to make up the overall "lead score". Just want to understand how your scoring fields are set up as it will impact how the smart campaign needs to be set up.