Can my employer, the sole contributor to my esop, contribute so much that it interferes with the 15% max I can put towards my 401k ?

Annual additon limits combine all defined contribution plans of the employer. The ESOP contributions could limit 410(k) elective deferrals if the other aggregate contributions to your accounts are large enough. If the ESOP is not leveraged, the limit for additions is 15% of pay. A leveraged ESOP (or a plan combination that includes a money purchase pension plan) can raise the limit to 25% of pay. In either case, the limit cannot be higher than $30,000. The rules for what constitutes an annual addition get complicated, especially for leveraged ESOPs.