for thoses interested , with devil spie (use google) I could manage to force specific application windows to pop up in the wanted workspace,
in the ~/.devilspie folder I crested a pidgin.ds file :
it forces pidgin windows to pop up in the 2nd workspace
I don't know well this application but I've seen we can set rules ... so I think there is way to make all windows of every applications pop in a same workspce