When I have a mono track and a stéréo output on this track.
I insert Guitar Rig (with mono input) I have stéréo with Guitar Rig. If I insert before (slot) a mono plug-in : ok.
But If I want to insert the plug-ins on a next insert slot, could it be mono ? (output GR will become mono ?)
I don´t really understand what you´re asking. but in a mono track any plugin´s output will be mono
Guitar Rig outputs are 2, I chosen 2 outputs on my mono tracks to benefit of the pan-effects (delay, reverb etc …)
So if I insert a plug mono on the insert 1 and Guitar Rig on the insert 2 ok.
But If I want to plug Guitar Rig on the insert 1 if I plug a mono in insert 2 Do I loose the pan of GR ?
I know I’m not very clear … 
OK, got it.
If Guitar Rig is on a mono track it will output in mono, no matter, if there is a mono plugin after it or not, and no matter if guitar Rig is set to be stereo.
If Guitar Rig is on a stereo track, it will output in stereo. If you insert a mono plugin after guitar rig on a stereo track, it will make the track output mono.