Is it expected that all pilots who go multi will eventually become captains or are some destined to remain co-pilots throughout their service.?
Goudie: in the same fashion that all RAF student pilots are recruited with FJ potential all ME students are expected to achieve captaincy at the end of their co-pilot's tour, ie: in only 3 years. At the end of this time they are 'boarded' for captaincy - these days it's a paper exercise but it used to involve wearing No 1s & sitting before a board comprised of SASO & a load of Sqn Cdrs being asked a bunch of questions to assess your suitability, on top of the paper exercise based on your reports of the past 3 years. Those that passed were considered suitable for a captain's slot but might get sent to CFS to be a QFI first, those that failed would invariably end up in a ground tour & in a few years have another go as a co again, possibly on a different type. Few failed to make it after their second go & most got captaincy after their first in my experience. So it's quite a short time to captaincy certainly when compared to civil aviation.
PN: what was the top slot for ME training graduates has varied considerably over the years & personal preference did seem to be a fairly significant factor. Strangely Tristar was deemed to be at the top when I passed through Finningley, Nimrods was volunteers mainly, perhaps due to the limitations of the location, although there seemed to be no shortage..... More recently C130J was the top slot as it was glass cockpit & Tac, closely followed by C17 (ditto but non-Tac) with C130K at the bottom. Not entirely sure where the order currently lies but I expect when ab-initio slots on the A400M open up that will take the top ranking.