What exactly does the option "Randomize Player Traits" when starting a new league with a custom roster file?
Does it change all existing players in the roster file, adding random traits?

And if the option is not selected, will new added player in the future get this traits?

Confirmed with Gary, it WILL randomize the player traits for whatever roster file you have in place, whether that's a custom file or a default one. The goal of that setting is to add some variability to those using the same file over and over again, so that's the case whether you use a custom one or not. (If you don't want that, I'd just leave it unchecked.)

It won't affect the new players being added, since they aren't included in the original roster file when creating the league.

Thanks for the info, that helps!!