Looking at the ruler that came with Icestorm and aftermarket "range cards" I see measurements in both centimeters and inches...yet only see inches mentioned so far in the beginner rules for Infinity. Are cm. used in some aspects of the game..or do some players use cm rather than inches?
To answer your actual question, you can use either metric or imperial, but you wouldn't use both in a game. If you look at the army builder https://army.infinitythegame.com/ you will see in the options you can select to print stats in CM or inches.