Yes, if the employees agree that the employer's contribution shall be restricted to 12% of Rs 15,000, you can do it.
In ESI, you cannot cover an employee whose salary is more than Rs 21,000. The employee whose salary exceeds Rs 21,000 should be excluded from the start of the next contribution period, and there is no option to continue to contribute. If you have employees with salaries over Rs 21,000 contributing to ESI, you should stop it because you cannot keep them under ESI. The system would accept the contribution because ESI is contributed based on the gross salary (minus some components like traveling allowance and washing allowance), but that salary shall include overtime wages as well. Thus, the ESI contribution salary may exceed Rs 21,000 (due to overtime wages), but if the salary without overtime wages and the allowances specifically excluded, like traveling allowances and washing allowances (permitted only if a uniform is provided), exceeds Rs 21,000, the employee will become uncovered from the next contribution period onwards.
If you have employees who are contributing to PF on a higher salary than Rs 15,000, that can be withdrawn only with the request of the concerned employee. This is because their contribution to PF will not cost the employer anything. Therefore, if an employee wishes that their PF should be deducted based on the actual salary, they can contribute it voluntarily.