The Clan Field Manuals.
"As a rule, Clan MechWarriors begin their careers in lighter-weight OmniMechs. Those who fail to demonstrate their worth to the Clan fall quickly into second-line or garrison ranks, where they can expect to pilot standard BattleMechs. Those who prove their value, by contrast, receive heavier and better OmniMechs as their achievements merit. At some point, most warriors discover an an affinity with a single 'Mech design or weight class, developing a particular expertise with it. Such affinities sometimes lead Cluster or Galaxy commanders to pilot light 'Mechs, though heavy or assault Omnis are far more common choices." P. 8, Field Manual: Crusader Clans, under 'Mech Assets. This section is reproduced in Field Manual: Warden Clans.
In terms of your examples, we don't see Jorge/Aidan's career prior to his arrival at Glory Station, having scored only the one kill in his Trial of Position and being defeated by a Warhawk but showing up next as a Star Commander. Having a command rank suggests he did something worthwhile between those points.
Bethany, and indeed Roar of Honour in general, are also poor examples; we know that Bethany was a good fighter but a misfit prior to being recruited by Angela, and so also may have been in something else. The rest of the novel is so riddled with inaccuracies (the Wolves being befuddled by their primary mode of warfare, the mobile campaign; the adaptable Ghost Bears and unable-to-innovate Wolves, and a MechWarrior who must be pushing two centuries if he was really involved in the development of the Kodiak...) and so a limited amount can be drawn from that.
It is also worth noting that the characters we see in novels are atypical.
