Adjustment of excess ITC

Hi experts,

My client started his business on 26th of Jan 2021.- Hyper mart

1. His GST tax liability for Jan month is  3900/- whereas his input tax credit is Rs.15600/-. My query is the entire ITC can be setoff against out put tax and the remaining can be c/f next month. 

2. whether he can go for composite scheme immediately or he has wait till April ?

Please clarify.

regards

P narayanan