Can I use a mercenary while I am shrouded?

If you shroud to another form using one of the shroud vendors in the Plane of Knowledge any active mercenary is automaticall suspended. You can unsuspend your mercenary once you are in your shrouded form, but it will be the same level as your new form (along with the corresponding level equipment and abilities).

When you shroud you won't have any money, which your will need to pay the mercenary. You should obtain some money before attempting to use your mercenary, or you may find them leaving you at a very inopportune time.