Confusion in term of itc

Not eligible. as you don't have your GST number for old firm

You need to file list of stock as on the date your threshold is crossed .you are eligible for only that stock which is held as on the date .As you need get registration effective from that date and need to pay GST on the supplies made from that date

Agree with Sanjith Sir's explanation...

As per the Provision of Section 18 sub section (1) clause (a) of the CGST Act 2017, a person who has applied for registration under this Act within thirty days from the date on which he becomes liable to registration and has been granted such registration shall be entitled to take credit of input tax in respect of inputs held in stock and inputs contained in semi-finished or finished goods held in stock on the day "immediately preceding the date" from which he becomes liable to pay tax under the provisions of this Act.

Warm Regards.

Yes once crosses thershold limit for registration, you should take GST Registration and File Form ITC-01 for taking input credit of GST paid on closing stock as on registration date.