Hi Laura
IMO it depends how much you want them shortened. If it's only a little bit like you said (less than 0.5 cm I'd say), then it should not be too much of a problem for the salon to file them down (a little). However, if you want to reduce the length any more significantly than this, then the apex (point of strength on the nail) will need to be moved accordingly as well, which requires them to take off any gel polish or nail art on your nails to reshape the nails, and then reapply gel polish etc.
If you only want them shortened a little and don't want to pay the salon, you can attempt to do it yourself, but here are a couple of pointers so you can (as much as possible) prevent service breakdown further down the line.
1) Stay away from the sidewalls. Someone inexperienced will easily file off too much of the sidewall, which takes away structure (it will break more easily).
2) Try to steady the nail at the cuticle (with another finger as much as possible, i know it can be awkward lol!) while filing the free-edge back and forth. This is to reduce vibrations from filing (vibrations can lead to lifting).
3) Acrylic is harder to file than gel, so you will probably need a lower grit file than 180.