I know i am going to make a very basic question, but: how to you manage all sound effects recorded on the dialogue mono track with the boom mic, do they stay there while everything else is rerecorded in stereo ? the issue is about creating a stereo image of all ambients effects etc (the final target is a 2.1 mix)
There is a small sample from the Purcell book available that covers PFX and guide tracks. It is available form the Focal site: http://www.focalpress.com/uploadedFiles/Books/Book_Media/Film_and_Video/Dialogue%20Editing.pdf
Well, you can't quite remove extra sounds from a dialogue track, unless by doing some cuts and fades. So yes, the dialogue track is fixed, unless you ADR it, which will give you a clean dialogue track. Therefore you should avoid picking up extra sounds that you don't want to be included in the dialogue track. You also mix everything else relative to the dialogue track. And the dialogue track is in mono, if it's recorded in mono, and you would likely keep it pretty much like that. With mono dialogue the stereo image is created with the other tracks, similarly like it's in music that includes vocals. Everything else is not necessarily recorded in stereo, because there are many ways for creating a stereo image. Also, in some cases mono sounds better, in some cases it's stereo that sounds better, and you pick the channel count according to what sounds better. Some tracks like ambiences as well as "sounds that move" are very often recorded in stereo though, for obvious reasons.
You might want to read the Book by John Purcell 'Dialogue editing...' which is about cutting and editing dialogue tracks.
If you cut out PFX from the dialogue track, you should fill that gap with roomtone. That way you can mix (and pan) those PFX independently from the dialogue track.