When to fork(copy) templates is a personal preference. As Boris says, an ad-hoc template is probably the *easiest* thing to do. But Kentico is a VERY flexible asp.net application, so you *could* find other ways to accomplish your objective - one other that I can think of is:
1 - Add an editable content control to the normal template and use a 'location' macro in the visibility field so that it is only visible on the one page where you need it:
{%cmscontext.currentdocumentparent.nodealiaspath|(contains)specialPage%}
So for this example, when you are on a page with 'specialPage' in the Url, the macro will return 'true' (which if inserted into the visibility field will show the control)
Be aware that in some scenarios this type of template could get covered in usercontrols that are only shown for a single page... which could make it difficult to debug and troubleshoot - hence the 'forking' comment.
Lance