I don't think there'd be any real reasons not to allow a trainee class, but unless this roleplay will include some sort of stat-based level up system, having a trainee class is sort of obsolete.

That said, someone might want to start off with a complete lack of weapon proficiency and develop that in the plot. For example a civilian might pick up a blade and 'class-change' to a myrm.
I don't really see why such a character would need to start as a '-1class' as they could just write myrm on their CS and point out their inexperience in the IC. But for people who prefer to select a class based on the early flow of things, it could work?

[hr]

To prevent a bombardment of 'Is this available' questions, it might be an idea to get out a list of available classes if possible. I personally prefer the position option, but that requires a bit more thought.