According to the interviewers, aircraft are assigned based on experience level, performance at the interview, and most importantly, operational need on the day groundschool starts.
No one gets hired purposely into the left seat. Everyone is hired into an FO position, however the situation at Mesa (on the Dash) has been such that by the end of ground school, new hires are able to bid Captain slots because there are not enough senior FO's bidding for them. As a result of this, they are type-rating new hires, and new hires are going straight into the left seat (if they bid it)
I hear that the same thing is happening / about to happen on the 1900.
Ray