Track cranks tend to be 165mm, so if you are running 1x then they can be a good option at a reasonable price. I use 165mm track cranks on my fixed TT bike - I basically refitted when moving from 172.5 ones (changed the bars too). I’m happy jumping between 165, 172.5 (road and 'cross) and 175 (MTB), but I have to remember to reset the crank length on my pedals (Powertap P1) if I move them around.
Shortening services need solid forged aluminium cranks, and they are typically done for use on recumbents (often to allow for a smaller fairing). This site says he only recommends shortened cranks for use with recumbents: Crank Arm Shortening
This has a list of available crank lengths from specialist manufacturers at the bottom - down to 120mm for some manufacturers (I think they are tandem/child cranks). Crank length and comfort for long-distance cyclists – Ride Far