you wont create another rule for this, you will need to edit the triggers for the existing shipping rules.
you will need to add the trigger:
subtotal for the entire cart contents subtotal price for the cart < 100
so that they only trigger if the weight is within the certain range AND the total price is less than 100.
the rules on the shipping tab are added up to the total shipping, Setting one rule to have a flat rate of 0 only adds 0 to the total of the other rules, it does negate them.