Bonnet,
this is my first airline so I don't know if this is normal at others but, just to make life simpler, we at Ryanair also have Second Officers.
Joining the company with less than 500 hrs on JAR25 A/C, you're initially a Second Officer, becoming a first officer after a full years service (minimum 500 hrs JAR25, which is no problem at Ryanair).
Two years after that you become Senior First Officer. Those two promotions give a 15% total salary raise each . Second Officers wear two stripes. FO's and SFO's three. No check or license requirements as far as I am aware. Then again, we probably don't have many Senior First Officers and I'll (possibly/probably) will never be one because you have a good chance of being a captain before you reach the SFO requirements.
As far as captains introducing first officers as "senior first officers' to the passengers, I reckon that sounds more comforting than telling them the first officer in question is on his first day on line with less hours than some of the passengers.
Hope this helps you a bit.
Good luck with persuading your company
Ziggy