It's a matter of selection from the GM point of view.
All the RM classes are possible in Shadow World, even the ones from RM VII and a few from Spacemaster.
After it's only a matter of will from the GM.
Do you want to have to deal with a character of this class or not ?
Once upon a time one of my gaming group was composed of :
- an Archemage
- a Chaotic Lord
- a Mentalist-Elementalist
- a Mentalist-Psionic
- a Montebanc
- a Sleuth
- a Fighter
and a few other I forgot.
There was only a few professions that were positivelly forbidden by our gamemaster.