templates

just to be sure-- i've just started working on a site thats just about 300
pages and 85 percent done. previous, templates werent used, everything was
handled by CSS. on the new few pages i am to create, i want to use templates.
this wont have any affect on the rest of the site, right? any adjustments or
additions to the template will only affect the new pages that i've added
generated from the template pages, correct?

thanks in advance!

correct.
although you can attach a template later to an already existing page but it often can lead to some confusion