I would like to launch some applications at startup, but I want them to be in a specific workspace.
I can edit the fluxbox startup-script to launch those applications, but I don't know how to specify in which workspace it must be launched.
you do not need any external tools, just read
man fluxbox-apps, edit
.fluxbox/apps and put there something like
[app] (name=xyz) [workspace] (n) [end]
gdevilspie to match this application and set the workspace.