There is no need to create portfolio pages or use a specific template. Just use specific URLs like –
http://yourdomain.com/?portfolio_category=inspiration
http://yourdomain.com/?portfolio_category=nature
which lists based on slug you specified for each portfolio category. In fact, if you have made the required changes, you can see these pages by clicking on the category links which pop up when you hover on portfolio items.
May be we need a menu or a sidebar for this to take care of navigation, but make sure you have everything setup where you see the above pages functioning before we setup some kind of navigation. At present the navigation will be through the portfolio hover only or through menu items which point to URLs like the ones I listed above.