I wouldn't put the manager in the military at all. Nor any dwarf with important skills. All it takes is one lucky arrow and you've lost the guy that keeps production flowing and have to train up a new one. Training a new manager from scratch in a busy fort can be painful. You queue up orders and they sit there for months with red X's by them, waiting for Urist McNewManager to figure out which end of the pencil is which.
I usually make my expedition leader my manager/broker/bookeeper, plus giving him mining duty. Early on he's not very busy with the administration, so he can be available nearly full time for mining. Later on, as the administration load increases, I'll probably have a few other miners trained up, so I'll turn off mining for him. He can go back to mining when I have a lot of digging to do, and then have it turned off again when a caravan arrives.
If/when the manager/broker/bookeeper dies, I often end up replacing hims with 2 or 3 dwarves, depending on who I have with appropriate skills. If I've got an immigrant with decent organizing, but no record keeping, and one with record keeping, but no organizing, then one becomes manager and the other the bookeeper. And I set up a new office because they both need one. Someone with some appraising/negotiating/judge of intent becomes the broker at that point.
My Chief Medical Droid is also set to mining at the start, since he seldom has anything to do. If there are injuries, it's easy enough to turn off mining for him. But he generally is available full time for mining and is always my first legendary dwarf (except for strange moods).
I hardly ever assign a sheriff/captain of the guard, and when I do, I usually regret it. I'd rather have a noble slightly miffed about someone going unpunished instead of half the fortress tantruming because a dozen popular haulers got executed for taking stuff to the depot that was export-forbidden after it was already traded.
The militia commander is just the first dwarf on the list of the first squad, as far as I can tell. No special duties that I know of. Like any other soldier, he's in a somewhat high-risk position and only available part time for other duties, so I wouldn't give him any important tasks that might be time sensitive.
Soldiers of all ranks are good candidates for masons, mechanics, wood cutters, etc. Any job where you want a bunch around, but skill doesn't matter too much for most of them. Just make sure you have a full-timer also doing the job. For example, have a primary mason and set the mason's shop to only allow novice or better. So the hoard of unskilled masons can crank out construction work en-mass, while the skilled guy is back at the shop making masterwork doors. Since soldiers are typically scheduled to be soldiering most of the time, and off duty some of the time, having a few more or less of them is no problem if they're doing unskilled labor. Or you can just give them no jobs and let them do hauling when they're off duty.