Agreed. Triggers are large predators and need at least a 6' 125 gallon tank. The maroon will eventually kill the percula. In saltwater only one of any similar fish. Percs are good for a 30 gallon. Maroons, as beautiful as they are, get to at least 6 inches and are mean. They will not tolerate any other clown, maroon or any other species.
As for favorite fish that are good in a 30 gallon. I love the orchid dottieback, I also love royal grammas but have never had success with them. Keep in mind body shape is an issue and only one fish with similar shape and/or coloring. I love shrimp gobies/watchman gobies. i wouldn't recommend a sand sifter gobie in that size tank. Not enough food and they make a mess.