I have a few templates that need to be placed within Joomla, and I'm not sure if Joomla supports SMARTY or if there is another way to integrate them? I don't want to have several parts of my web site running with different logins if I can use Joomla for everything.

The closest thing to what you're describing that I'm aware of is the WTElite engine, which allows those templates to be installed and parsed. Though I'm not sure how well it supports authorization.

extensions.joomla.org/component/option,com_mtree/task,viewlink/link_id,1370/Itemid,35/