Legal Question in Employment Law in California

I have no idea what you mean with "pre-adverse and adverse letters", but all employment is 'at will' in CA, unless you have entered into a binding written employment agreement. If you have, then the terms of the agreement determine your remedies. If not, and have only issued an offer letter, then you can terminate the employment or the offer at any time. The primary exception to that is if you offer a job and the new employee moves from out of area in reliance on that offer. He would then have a legal claim for breach or fraud, if he could prove his facts.