As long as it would make sense for them to be in any way affiliated with the League or even exist in the DCAU. That's the general operating principle for any character really. Shade the Changing Man, Icon, and Amethyst could exist in the RP because they fit those criteria and also fit the tone and atmosphere of the DCAU.

Or alternatively, characters and their background can be altered to better fit in and connect with the DCAU like Amazo's or Hawkman's alternate origins in the show.