First thing, make a back up copy of any and all files you plan on tinkering with.
Yes, you can adjust the reticles without it interfering with your online game play.
You can adjust the original files. This type of change will show up only on your PC, wether or not your playing single player, or multi-player. Even if you host the MP game, the people who join it will not have to have the same alterations. Like I said this will only show up on your PC. Something to consider though, if you choose this route without makeing a backup of the file you can permanetly (unless you reinstall the