Minimum tenancy term before a s21 can be actioned.

Possession - Section 8 and Section 21

Hi
I may need to move a tenant from one property to another and am trying to confirm what is the shortest duration of a fixed term tenancy agreement before a s 21 can be actioned, rather than served.
Is it 6 months, or can I give a 2 month, 1 week fixed term and issue a s21 on the day after the tenancy has started.
It's a long story about why I would want to do this, just checking out my options to move a tenant quickly who is causing a problem in one house and then end their tenancy with me totally. They seem will to move house, so this may be a less traumatic route rather than keeping them in the current house.
Advice gratefully received. Many thanks