Ha! Although it's one of my favorite fish, I have to take issue with the "peacable" part! 8O C. argi is easily one of the most, if THE most aggressive of the centropyge. They are known for this, and my experience as been no different. I'm talking a dozen or so specimens from my personal experience over the years. My current one is 3 years old, and the boss of his tank. Don't get me wrong, they are GREAT fish, and very hardy! I love those little guys, tons of personality too.
However, I would not risk keeping them with other dwarf angels unless the tank is very large, say 180 gallons +. They should usually be the last fish added when speaking of a mixed community, but when adding with other dwarfs, you just roll the dice. It can be done in smaller tanks, I've done it before as well, but I wouldn't recommend this to someone else necessarily.
Sometimes you just get lucky like Matt for instance, but this experience should in no way be cause to call the species as a whole mellow! I've experienced a calm clown trigger, but I realize that's the exception rather than the rule. C. argi is a little fireball of a fish, and you should assume that it's going to be aggressive in your setup. If you get lucky, great! Just don't PLAN on it.
Cheers
Jim