I usually handle ranks, at least with Mechwarriors and pilots. Lately I've been doing more. One of the problems was difficulty in determining who was going to be in command of a formation - infantry commanders were frequently coming out ahead of all others due to being at the company level compared to lance/platoon level for Mech/armored units. Also important as ranking officers determine certain bonuses from Tactics, Strategy, and Leadership skills. So I tossed out a few conventions, so that the branches aren't entirely equal until they start hitting 'command' ranks, around Lieutenant Colonel or so. Also added in numerous intermediate ranks where they made sense, and some appropriate ranks for the technical side.
Pretty much everyone new starts at recruit, except for infantry hired as a unit which gets standard distribution with 2nd Lt for CO. Everyone is promoted to first enlisted rank after a year of no combat, or after first combat. Private 2nd gets PFC'd after another year or their unit does well in combat. Promotion to corporal happens if the person does very well. Promotion to Sergeant and above is space limited and regardless of skill rating or time in service/grade e.g. Mech lance have one as a second-in-command, infantry have one per squad (although a Corporal might fill in if there aren't enough). Vehicle commanders are similarly sergeants, with corporals if needed. I field oversized infantry formations to better match generated OpFors, with the senior being (infantry) Captain or 1st Lt, with the rest typically 2nd Lts.
Large vessel crews are still in a 'not great' status. Captain as who... pilot? 'Crewmember'? Pilots as officers as traditional, or enlisted? Similarly, technician promotions are somewhat more 'feel' based, some being organized into squads (either rifle or custom infantry) with appropriate leadership structure, while others without small arms skills are not.
Promotion checking is typically handled at the end of shorter contracts, and monthly on longer ones like garrison. Or when a new formation is stood up or someone retires (which I handle manually, and highly recommend doing so over the automated version), when there is a chain of promotions to fill empty command slots.
Everything below is based on my personal understanding of Enlisted, Warrants and Officers (Line, Staff and Restricted) in the US Navy. Your milage will vary...
Aerospace pilots (presumably) are going to be Officers (If you go ATOW pathways as a guide) but in my current Megamek game setting, sometimes Pilots are enlisted who
grew up learning the stick with commercial spaceborne aircraft (if you take the time to type up a bio for specific toons you acquire).
Mechwarriors, Tankers or all Infantry subtypes would commonly be enlisted (unless in your mind's eye, you want the prospective toon to be an officer, either starting out as an Officer or later conversion into an officer from the enlisted ranks).
VTOL pilots would be Warrants (as they are in the US Army aviation aspects of the service) but you could also make Conventional Pilots Warrants (in order to make that line of thought more... IDk... fluid?)
Dropship, Jumpship and Warship crews are distinct different and here i go simply along my Naval experiances. Gunners will be enlisted (i chose to call them Fire Controlmen like the job in the Navy. They look at scopes, push buttons and bark into mics, where i assume nameless someones actually Ready, Shoot, Aim), Technicians are Engineer's Mate (Dropships) and Enginemen (Jumpships). The pilots?
One pilot is the 1st Officer (and is an Officer) the second is the 2nd Officer (also a O) and the third will be a Quartermaster (enlisted) (again, going by my understanding of things Space Navy). Using ranks is important because the way megamek works, if your highest rank (effectively) is the enlisted pilot, its his/her name that appears on the ToE tab, not the intended officer
Jumpships is the same with on addition: I call the Jump Navigators... Navigators (and they are officers in my mind's eye)