What should my traffic settings be? Can I use a traffic injector?

It actually doesn't matter. You can use your favorite traffic injector, or you can use the built-in traffic.  Flight followings / radar services will give you traffic advisories for any aircraft that happen to cross your path, just like in real life. It's really cool!

At this moment, there is no other ATC service relating to other traffic. We are working on traffic injection, projected to become online at the end of 2024. From that moment, you will be able to see the other multiplayer aircraft around you and may be even more.