Add a column to your cart to store the category if it doesn't exist already. I'll call that column Category in my example but use your existing category column if you have it already....
Create a calculation in your cart: DiscountedTotal = [TotalPrice] * (intval([Category]) < 7)
Then create a discount rule When the total of DiscountedTotal > 0 discount Amount total of DiscountedTotal * 0.2