LiberatorB24, promotional seniority doesn't take qualifications into account at all... Preferences are completely up to the individual, and whether they have the seniority for the position or not depends on how popular that position is at the time.
The 767 used to be the most junior (conditions weren't fantastic at the time), now with fewer slots and better packaging it's more senior than the 737. The A330 is the most senior, short of the 400. Off the top of my head A330 FOs have about 7 years seniority, and 400 FOs are closer to 10.
People getting FO slots this year have 2.5 years seniority. Two years ago you could do it in less than a year!
The JSA cadets will have the advantage of seniority two years ahead of their start date. That's a can-of-worms in itself! A freeze period as an SO would be the easy solution.
Lancer