ECA quota or Extra Curricular Activity quota is one of the most competent seats students are rushing into in our current situation. As the number of seats are limited and the quota focuses on the inherent talents in the aspirant rather than academic merit, you might require a letter of recommendation, attested by the Principal of the institution you have been attending the last, provided a detailing on the activities you took part in. You will have to submit a separate application particularly to the colleges that offer your desired course rather than applying for the general merit as very few courses are offered through ECA quota.

As you have been particularly asking, I would suggest you to give separate application for the common application and for the ECA quota. The ECA quota can give 15% relaxation to the academic score even though you will have to go through two auditions, preliminary trial and the final trial infront of the experts, on the scheduled dates they will instruct you on. The stiff competition for the 5% quota doesn't provide you with the arrangements and facilities that you need to perform, which you have to arrange individually according to the guidelines of the institution. 

So inorder to mark yourself in case of necessity apply for both the admission procedures separately. The students applying on ECA generally aim for the relaxation of marks, which they want to compensate in the tight competition. Your confidence in considering to apply for both the quotas seems appreciable and it gives you an upperhand among the competition to get admission for the desired course.

I wish you all the best.