S21 dates and S8

Hi My T was on a AST and 6 month came to an end and is now on periodic rolling contract. I've issued the T with a S21, the start of rental period is on the 7th of the month. I issued this in person on the 27th Nov so that date i put on the S21 was the 7th Feb - is this valid?

Also I am thinking of issuing a S8 due to rent arrears - can i issue one now alongside the S21 and what are the merits of doing so? - should i just go with county court action if rent arrears are not paid by 7th Feb?

Thanks!