Legal Question in Business Law in California

Perhaps.......it really depends upon what you mean by "make.....mandatory." In general, and subject to any limitations imposed on it by a union or personal contract, and employer can make forward-looking rules affecting your duties and your pay. An employer cannot reduce your pay or make rules that have retroactive effect, at least not without your consent. There may be some other limitations on the employer's rights in various statutes and Labor Commissioner rules, but basically the employer can tell you what the job conditions are in the future.