Please kindly note that Microsoft Q&A forum recommends asking one question per thread so we can better focus on one question.
It would also make searching answers easier for other community members.
In this case I would suggest we mainly focus on the permissions of Team and channel creation question.
For Data Loss Policies questions, please consider creating a new thread.
Thanks for your understanding.
I would like to add this group to the Teams Admin Center such that only this group 'A' members can create Teams and their Channels.
The permission to create new Team (in another word Microsoft 365 group) is associated and controlled by Azure Active Directory.
For more information please refer to this link: Manage who can create Microsoft 365 Groups
It is not limited to Teams, but also affects other services:
If you would like to restrict users from creating new Teams, you may need to follow the steps in the link to configure the setting in Azure AD.
Well again note that it also affects Microsoft 365 group creation in other services.
The permission to create new Team Channel in a Team can be managed by Team owners in Teams.
Please see the following screenshot:
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