Is it nessesary to choose mode quick or expert before I am making a show? or is it possibly to start

"Another question: is it a god idea to make rendition along the produktion or first when you are finished with the edit?"

I am assuming you mean using the "render" feature during the editing process.  Rendering is only necessary if the editing preview becomes "sluggish" or "jerky".

You can go from Quick to Expert at any point, but it's not so easy to go from Expert down to Quick.

Expert mode employs tools and timeline features not always available in Quick.