Ask for them to be hemmed (the shortening part) then tapered (the 'keeping it skinny' part).
I'm not sure if they can make alterations if there's only like 1-2cm of material to work with, if you know what I mean. Like, it would be way easier to taper bootcut than something that is already tapered. But that doesn't matter if you're taking like 5+ inches off the inseam or something.