Hi,
The point of templates is to preserve exactly the same settings across all the documents that use them. Therefore what you are asking goes directly against their functionality. However you have several options you could consider:
Create a copy of the template, assign it to a document and modify the web part properties. You can either clone the template in
CMS Site Manager -> Development -> Page templates, or create ad-hoc copy for a single document in the
CMS Desk -> Content -> Edit -> Properties -> Template.
Other solution could be to use widgets, for which you can define a default layout and properties in Style tab and modify them for each document in Page tab. However, you should be aware of
limitations of using widgets.
Regards,
Josef Dvorak