The first two fish I added to my 75 gal were a small Maroon Clown and a Blue Devil Damselfish. No issues, seems the tank was large enough for both of them and since they were about the same size, they didn't harass each other. That was a year and a half ago. Since then I've added another Maroon and now have a pair hosting a BTA I added before introducing the second clown. Also have a lawnmower blenny, mandarin dragonet, yellow tang, singapore angel and one blue/green chromis (originally had 3 of those). Even though the maroon was one of the first additions, there are no major issues and everyone gets along reasonably well. The blue devil damsel has on occasion taken a bit of fin off other fish, but he hasn't grown much and is one of the smaller critters currently in there. The female clown only gets irritated if something gets too close to the BTA. No one bothers the dragonet, they have some coating making them unpalatable to other fish. Once in a while the singapore angel gets a little boisterous, but never really harms anything. The yellow tang is the biggest dude in the tank and no one challenges him. He's very well tempered and doesn't bother any other inhabitants. The odd time the tang and angel will have a minor bumping bout, but it's usually harmless.
Anyhow, from personal experience, I added a maroon clown right at the start and she's not overly aggressive.... yet. The pair hasn't spawned that I'm aware of, so that may eventually make her meaner. I've read that as they age, maroons can get nastier, but for me, so far so good.