I would like to have some questions in my survey grouped (so I can ajax the follow-up questions) and other not. How can I achieve this?

Place the "single" questions in separate groups.

As Tpartner says.

Unfortunately at the moment, there is no "page" concept in Limesurvey. If you want to create surveys with more than one question on a page, you need to use "group by group" and use the question groups as a substitute for "pages".

Would be great if we could have both one day, visual groups (aka pages) and logical groups (aka the current question groups).