There is legally no issue if you have two sets of employees: one set contributing based on their actual salary and another contributing Rs 15,000. This decision depends solely on the employer.
You can set the salary of new employees at a level higher than Rs 15,000. However, the question is what happens if the PF qualifying salary is increased to Rs 25,000 or Rs 30,000 by the EPFO? Discussions are ongoing to revise the threshold limit from Rs 15,000 to Rs 30,000. It is expected that both under ESI and EPF, it will be increased to Rs 25,000, mainly because in almost all states, the statutory minimum wage exceeds Rs 15,000.
Even if you set the salary at Rs 15,500 or Rs 16,000 to exclude them from PF coverage, the main question is whether there would be any bifurcation of salary like basic wages, HRA, conveyance, etc. If you are contributing based on basic pay alone, then to exclude the new employees from the ambit of PF, you should make the basic pay more than Rs 15,000, right? Then, will it be higher than the basic pay of existing employees? In compensation restructuring, I have encountered such issues as well. Therefore, setting the salary with the intention of avoiding statutory coverage is not a wise decision.