I think that should be correct. However, you should be aware that some of the extensions don't write the easiest to follow mark-up or CSS code (form builder I'm looking at you). When creating fluid grid layouts the default doc type is HTML5 - you can change this, but if you want to use HTML 5 and adhere to it's markup rules you'll need to do some fairly extensive work.
All fluid grid layouts really are is a set of media queries already embedded into a page.
You'll need to take the original css code generated by the extensions and then add in new values for the mobile/tablet devices yourself in the CSS file you create when choosing the fluid grid layout system. Making sites responsive is not a 5 minute job, its quite time consuming and you need to consider your CSS a lot more.
Hope this helps you.
