$1:
like in the Canadian military you can have a Navy MP, Army MP or an Airforce MP.
The Canadian military only has one type of meathead(j/k...when I enlisted there was a 2 yr waiting list for the MPs). I think they did/do their training in Borden and are assigned to various bases. The same goes for stewards, cooks, supply techs, clerks, met techs and many other trades necessary to keep various operations running smoothly.
When I was doing my TQ3 and TSQAB at Stadacona and Windsor Park we had quite a few remusters from the infantry(usually Chicken Fuckers). Some became firefighters and the others usually became a bos'n.