There was a military helo guy in my new hire class that was brought in as a street CA. It was on the Beech so it is very similiar to your C-12 and would likely be an easy ground school.
What everyone says about the FO's bumping you as they upgrade is true, but when the FO's upgrade the Street CA's will have already logged about 500 hours a year of PIC and therefore in my mind are a step ahead in the game. Anyone claiming anything else about it is better to be FO first, stop kidding yourself are you going to pass up your chance to upgrade just so you can be a more senior FO. Alright then ....Most ppl I hear this from just don't have enough time to upgrade. Let's not also forget once you have that seat, there needs to be a vacancy for that senior FO to upgrade. All that said, being a street CA is now easy task. Your QOL will definately be in the dumpster for a while and passing a type ride in an a/c you have never flown is no cake walk. Remember that it looks bad to fail a 121 training event. If you are attempting your first job in the 121 world as a street CA, all I can say is you have balls. I did this and it was quite daunting even though I had over a 1000 PIC in a similiar a/c so all I had to worry about was this new world of 121 ops...I still sweated my way through that training. Well thats about it, Good Luck to all in your endeavors.