I generally start a campaign at the company level, and do my initial staffing by turning off Campaign Options / Finances / Pay for personnel recruitment, then doing a "Marketplace -> Hire Personnel in Bulk" for Mechwarriors. I hire 16 for 12 mechs, switch the Personnel view to Tactical Skills, and tap the people with Tactics skills to be the initial officers (lance commanders). On the very rare occasion that someone starts with a Strategy skill, they get tapped to be the company CO, otherwise I pick among the tacticians to promote. This is a highly specialized selection based on how cool their name sounds, what their generated portrait looks like, their P/G skills, etc. Having a couple extra Mechwarriors in the initial cadre allows me to screen for additional skills like Medical (Congrats, Doc) or Admin. Leftovers who don't have a Battlemech to ride or some other useful function get sacked.
I initially get Aero Pilots and vehicle crews by doing a Hire full complement on their platform in the hanger. I get support staff by purchasing a platoon or two of conventional infantry then editing them into Mechanics, Mech and Aero Techs, Administrators, etc. by setting their tech/admin/whatever skill points equal to whatever their small arms points were.
Once I've got a fleshed out company, I switch personnel recruiting costs back on and live by the AtB rulesets.