I looked into it and couldn't figure it out. The only thing I could think of was if you made a custom race called "all except festron" and included all the citizen portraits from all citizens and leaders from the races you wanted included. It would just be cosmetic as your citizens would all be the same race (your custom race) but on the planet screen it would look like there's a bunch of different ones. They would all have the stats and modifiers you defined for that race so a citizen with a portrait of korath wouldn't have an approval drop for a citizen with a portrait of another race because in your mod they would technically be the same race. So it would just be about appearance and have no depth to it.
If you can find the option somewhere you might be able to exclude race trait consuming or race festron but I couldn't find it anywhere.