You can't set anything except soul soil on soul_fire in creative with commands.
In 20w08a you could add interesting contraptions with soul_fire, but it is not available in 20w09a. I consider it as regression as I don't see a reason to disable this for creative players.
Screenshot from 20w08a attached to show creativity example which is not possible in 20w09a.
Also take note that you can set normal fire on soul_soil with command.