Is it possible to assign different sets of extensions for different website templates.

For example,

Website Template A allows installation of banners, Latest News,...


Website Template B allows installation of Forums, Virtuemart...


Thanks.

With JMS, you can not restrict the installation of extensions but you can pre-cnfigure a website A with the extension you want and later use it in the website template A.
Idem for B.

When a website is created it is still possible to install additional extensions that are alreay present in the master website.
For example, you have created a website from the template A, it will still be possible to install the forum, Virtuemart or whatever that exist on your master website.

In that case, can I limit the rights of "owners" of slave websites by assigning the backend user type "Manager" to them rather than "administrator".

In other words, I don't want the users who bought slave websites to install any extension themselves.

Thanks.

YES, you can define any special user that have access to the back-end.
Manager is the one that have the less permissions.