Dear Jabin 

Candidates will need to produce the required documents at the time of reporting to the college allotted in the counselling. Now, the documents you will be required to produce will depend upon in which counselling you have been allotted the seat. 

For AIQ: If you get admission under All India Quota, then,as per the documents required previous year, you will not be needed to take migration certificate to the allotted college.

For admission through state counselling (in private college of Tamil Nadu): Now, given that you are from Assam, you will not be eligible for state quota admission in Tamil Nadu. However, if you get admission in private college of Tamil Nadu, then as well, as per the documents which were asked previous year, you will not be required to produce any migration certificate.

Besides that, an individual college may ask you for the migration certificate, so it is always better to have that. But still in case, you do not have it right now or it seems you will be able to get it sometime later, then do not worry much about it, since you should get an appropriate window from the allotted college to produce the same. Migration certificate isn't as important a document as would be your Class 12 marksheet/certificate, rank letter, allottment letter, etc.

Hope it helps. 

All the best!