ESIC is compulsory where the coverage is applicable (check with the ESIC office to see of your sites are covered)
There is no option of not covering them.
For all those not covered, both for locations outside ESIC coverage and those with wages above ₹15000, you are open to risk of compensation for any accident. So for those employees you need to take the workman insurance cover. Remember that any other insurance like group cover, accident cover, etc is not going to cover compensation under employee compensation act. Courts have already given judgements clarifying that any other cover like accident insurance is additional benefit to the employees and the compensation under WC is still applicable. Only workman compensation act insurance cover which specifically reimburses the compensation under the act will be adequate.
Incidentally, ensure you are paying cess under BWOC act also