Your 100% het guarantee is directly related to how honest the person you are buying them from is. Anyone can take a normal, call it a 100% het and sell it as such. If you know the person you are buying them from, and trust them, the guarantee is much better than buying a "100% het" from a stranger on Craig's List.
You are not guaranteed to produce at least one clown from a pairing of 100% hets. Each egg has a 25% chance of being a clown. Statistics being what they are, there is a chance you get zero full clowns.