I'm using Captivate 2019 (without updates).
Is it possible to Save a cptx as a cplx for responsive design?
Can I make a Template that is responsive Design (fluid boxes)?
Captivate templates (CPTL files) haven't had a good reputation for being bug-free in recent years. So most experts on this forum would probably not be recommending you use anything other than a CPTX file as a starter 'template' for new projects. The good news is that an ordinary CPTX file is fine to use as a template, but to ensure that unwanted changes are not made to the template itself, all you need to do is set its file properties to READ ONLY. You do this in Windows Explorer, not Captivate.
So, just set up a responsive fluid box project the way you want it, mark it as READ ONLY, and then when you open it and try to save any changes, you'll be prompted to SAVE AS. Give the file a new name, and then you have a new project based on the previous one.
Totally agree with Rod about using a cptl file. Although theoretically it is possible, too buggy.
A common misunderstanding is that you need a template for design consistency. In Captivate all design resides in the THEME: color palette, object styles, master slides (including eventually shape buttons with advanced or shared actions), skin setup and Recording defaults. Knowledge about the exact role of Themes has become more and more important since the release of 11.5 where you can use several themes in the same project.
Have a look at:
Lot more to explore about Themes on my blog.