I ran across several old, incomplete questions posted on the subject of the very undocumented simple portfolio plugin using archive.php for the default category template and questions on how to change this. For anyone searching on this topic in the future, I thought I would post the answer here which all worked for me.
If you are trying to display an archive of category posts, create a template called taxonomy-portfolio-categories.php and this will override the wordpress use of archive.php which may interfere with an existing blog etc.
If you wanted to show a list of posts by client in a custom template, do the same thing but name it taxonomy-portfolio-clients.php
I really enjoy this plugin but coding the templates can be a beast without documentation.